diff --git a/src/codemods/annotate_exports.ml b/src/codemods/annotate_exports.ml
index 4abadf99a8d..83646aa09af 100644
--- a/src/codemods/annotate_exports.ml
+++ b/src/codemods/annotate_exports.ml
@@ -96,6 +96,7 @@ module SignatureVerification = struct
       (fun acc err ->
         match err with
         | Type_sig.CheckError -> acc
+        | Type_sig.BindingValidationError _ -> acc
         | Type_sig.SigError err ->
           let open Signature_error in
           let (tot_errors, acc) = acc in
diff --git a/src/parser_utils/file_sig.ml b/src/parser_utils/file_sig.ml
index 8cde5c0f615..75eeeca6aa7 100644
--- a/src/parser_utils/file_sig.ml
+++ b/src/parser_utils/file_sig.ml
@@ -59,7 +59,10 @@ and require_bindings =
   | BindNamed of (Loc.t Ast_utils.ident * require_bindings) list
 [@@deriving show]
 
-type tolerable_error = SignatureVerificationError of Loc.t Signature_error.t [@@deriving show]
+type tolerable_error =
+  | SignatureVerificationError of Loc.t Signature_error.t
+  | SignatureBindingValidationError of Loc.t Signature_error.binding_validation_t
+[@@deriving show]
 
 type tolerable_t = t * tolerable_error list
 
diff --git a/src/parser_utils/file_sig.mli b/src/parser_utils/file_sig.mli
index 8709dab6e2a..3ed18b05cfb 100644
--- a/src/parser_utils/file_sig.mli
+++ b/src/parser_utils/file_sig.mli
@@ -101,7 +101,10 @@ and require_bindings =
   | BindNamed of (Loc.t Flow_ast_utils.ident * require_bindings) list
 [@@deriving show]
 
-type tolerable_error = SignatureVerificationError of Loc.t Signature_error.t [@@deriving show]
+type tolerable_error =
+  | SignatureVerificationError of Loc.t Signature_error.t
+  | SignatureBindingValidationError of Loc.t Signature_error.binding_validation_t
+[@@deriving show]
 
 type tolerable_t = t * tolerable_error list
 
diff --git a/src/parser_utils/signature_builder/signature_error.ml b/src/parser_utils/signature_builder/signature_error.ml
index 46cae833de7..9c264a9579d 100644
--- a/src/parser_utils/signature_builder/signature_error.ml
+++ b/src/parser_utils/signature_builder/signature_error.ml
@@ -15,4 +15,6 @@ type 'loc t =
   | UnexpectedExpression of 'loc * Flow_ast_utils.ExpressionSort.t
 [@@deriving show, iter, map]
 
+type 'loc binding_validation_t = NameAlreadyBound of 'loc [@@deriving show, iter, map]
+
 let compare = Stdlib.compare
diff --git a/src/parser_utils/type_sig/__tests__/type_sig_tests.ml b/src/parser_utils/type_sig/__tests__/type_sig_tests.ml
index 64f300a11bc..cd2fffdaf0a 100644
--- a/src/parser_utils/type_sig/__tests__/type_sig_tests.ml
+++ b/src/parser_utils/type_sig/__tests__/type_sig_tests.ml
@@ -5245,6 +5245,108 @@ let%expect_test "builtin_declare_namespace" =
     Builtin global value globalThis
     Builtin global value ns |}]
 
+let%expect_test "declare_namespace_declaration_merging" =
+  print_builtins [{|
+    declare namespace ns_v {
+      declare const a: string;
+    }
+    declare namespace ns_v {
+      declare const b: string;
+    }
+    declare namespace ns_t {
+      type T1 = string;
+    }
+    declare namespace ns_t {
+      type T2 = string;
+    }
+    declare namespace ns_v_and_then_t {
+      declare const a: string;
+    }
+    declare namespace ns_t_and_then_v {
+      type T1 = string;
+    }
+    declare namespace ns_v_and_then_t {
+      type T1 = string;
+    }
+    declare namespace ns_t_and_then_v {
+      declare const a: string;
+    }
+
+    declare const non_ns_value: string;
+    type non_ns_type = string;
+    // The following namespaces won't have any effect
+    declare namespace non_ns_value {
+      declare const b: string;
+    }
+    declare namespace non_ns_value {
+      type T1 = string;
+    }
+    declare namespace non_ns_type {
+      declare const b: string;
+    }
+    declare namespace non_ns_type {
+      type T1 = string;
+    }
+  |}];
+  [%expect {|
+    Local defs:
+    0. Variable {id_loc = [2:16-17]; name = "a"; def = (Annot (String [2:19-25]))}
+    1. NamespaceBinding {id_loc = [1:18-22];
+         name = "ns_v";
+         values =
+         { "a" -> ([2:16-17], (Ref LocalRef {ref_loc = [2:16-17]; index = 0}));
+           "b" -> ([5:16-17], (Ref LocalRef {ref_loc = [5:16-17]; index = 2})) };
+         types = {}}
+    2. Variable {id_loc = [5:16-17]; name = "b"; def = (Annot (String [5:19-25]))}
+    3. TypeAlias {id_loc = [8:7-9]; name = "T1"; tparams = Mono; body = (Annot (String [8:12-18]))}
+    4. NamespaceBinding {id_loc = [7:18-22];
+         name = "ns_t"; values = {};
+         types =
+         { "T1" -> ([8:7-9], (Ref LocalRef {ref_loc = [8:7-9]; index = 3}));
+           "T2" -> ([11:7-9], (Ref LocalRef {ref_loc = [11:7-9]; index = 5})) }}
+    5. TypeAlias {id_loc = [11:7-9];
+         name = "T2"; tparams = Mono;
+         body = (Annot (String [11:12-18]))}
+    6. Variable {id_loc = [14:16-17]; name = "a"; def = (Annot (String [14:19-25]))}
+    7. NamespaceBinding {id_loc = [13:18-33];
+         name = "ns_v_and_then_t";
+         values = { "a" -> ([14:16-17], (Ref LocalRef {ref_loc = [14:16-17]; index = 6})) };
+         types = { "T1" -> ([20:7-9], (Ref LocalRef {ref_loc = [20:7-9]; index = 10})) }}
+    8. TypeAlias {id_loc = [17:7-9];
+         name = "T1"; tparams = Mono;
+         body = (Annot (String [17:12-18]))}
+    9. NamespaceBinding {id_loc = [16:18-33];
+         name = "ns_t_and_then_v";
+         values = { "a" -> ([23:16-17], (Ref LocalRef {ref_loc = [23:16-17]; index = 11})) };
+         types = { "T1" -> ([17:7-9], (Ref LocalRef {ref_loc = [17:7-9]; index = 8})) }}
+    10. TypeAlias {id_loc = [20:7-9];
+          name = "T1"; tparams = Mono;
+          body = (Annot (String [20:12-18]))}
+    11. Variable {id_loc = [23:16-17]; name = "a"; def = (Annot (String [23:19-25]))}
+    12. Variable {id_loc = [25:14-26]; name = "non_ns_value"; def = (Annot (String [25:28-34]))}
+    13. TypeAlias {id_loc = [26:5-16];
+          name = "non_ns_type"; tparams = Mono;
+          body = (Annot (String [26:19-25]))}
+    14. NamespaceBinding {id_loc = [0:0];
+          name = "globalThis";
+          values =
+          { "globalThis" -> ([0:0], (Ref LocalRef {ref_loc = [0:0]; index = 14}));
+            "non_ns_value" -> ([25:14-26], (Ref LocalRef {ref_loc = [25:14-26]; index = 12}));
+            "ns_t_and_then_v" -> ([16:18-33], (Ref LocalRef {ref_loc = [16:18-33]; index = 9}));
+            "ns_v" -> ([1:18-22], (Ref LocalRef {ref_loc = [1:18-22]; index = 1}));
+            "ns_v_and_then_t" -> ([13:18-33], (Ref LocalRef {ref_loc = [13:18-33]; index = 7})) };
+          types =
+          { "non_ns_type" -> ([26:5-16], (Ref LocalRef {ref_loc = [26:5-16]; index = 13}));
+            "ns_t" -> ([7:18-22], (Ref LocalRef {ref_loc = [7:18-22]; index = 4})) }}
+
+    Builtin global value globalThis
+    Builtin global value non_ns_value
+    Builtin global value ns_t_and_then_v
+    Builtin global value ns_v
+    Builtin global value ns_v_and_then_t
+    Builtin global type non_ns_type
+    Builtin global type ns_t |}]
+
 let%expect_test "builtin_pattern" =
   print_builtins [{|
     const o = { p: 0 };
diff --git a/src/parser_utils/type_sig/type_sig.ml b/src/parser_utils/type_sig/type_sig.ml
index 41851efe810..4894974299d 100644
--- a/src/parser_utils/type_sig/type_sig.ml
+++ b/src/parser_utils/type_sig/type_sig.ml
@@ -615,6 +615,7 @@ type 'a op =
  *)
 type 'loc errno =
   | CheckError
+  | BindingValidationError of 'loc Signature_error.binding_validation_t
   | SigError of 'loc Signature_error.t
 [@@deriving show { with_path = false }, map]
 
diff --git a/src/parser_utils/type_sig/type_sig_mark.ml b/src/parser_utils/type_sig/type_sig_mark.ml
index 9922c60f9bd..123bd645d0f 100644
--- a/src/parser_utils/type_sig/type_sig_mark.ml
+++ b/src/parser_utils/type_sig/type_sig_mark.ml
@@ -266,6 +266,8 @@ let mark_exports
     SMap.iter (fun _ t -> mark_export ~locs_to_dirtify t) names;
     List.iter mark_star stars
 
+let mark_errors = List.iter (Signature_error.iter_binding_validation_t (mark_loc ~visit_loc:ignore))
+
 let mark_builtin_module (loc, exports) =
   mark_loc ~visit_loc:ignore loc;
   mark_exports ~locs_to_dirtify:[] loc exports
diff --git a/src/parser_utils/type_sig/type_sig_mark.mli b/src/parser_utils/type_sig/type_sig_mark.mli
index 8b6fccc64a1..d17e543c95a 100644
--- a/src/parser_utils/type_sig/type_sig_mark.mli
+++ b/src/parser_utils/type_sig/type_sig_mark.mli
@@ -13,5 +13,8 @@ val mark_exports :
   Loc.t Type_sig_parse.exports ->
   unit
 
+val mark_errors :
+  Loc.t Type_sig_collections.Locs.node Signature_error.binding_validation_t list -> unit
+
 val mark_builtin_module :
   Loc.t Type_sig_collections.Locs.node * Loc.t Type_sig_parse.exports -> unit
diff --git a/src/parser_utils/type_sig/type_sig_pack.ml b/src/parser_utils/type_sig/type_sig_pack.ml
index c3dc209c2f8..62dc6732432 100644
--- a/src/parser_utils/type_sig/type_sig_pack.ml
+++ b/src/parser_utils/type_sig/type_sig_pack.ml
@@ -235,7 +235,7 @@ type 'loc pattern =
 
 type 'loc cx = { mutable errs: 'loc errno list }
 
-let create_cx () = { errs = [] }
+let create_cx errs = { errs }
 
 let pack_smap f map =
   let bindings = Array.of_list (SMap.bindings map) in
diff --git a/src/parser_utils/type_sig/type_sig_parse.ml b/src/parser_utils/type_sig/type_sig_parse.ml
index 3d75ba93274..7e23b72e56a 100644
--- a/src/parser_utils/type_sig/type_sig_parse.ml
+++ b/src/parser_utils/type_sig/type_sig_parse.ml
@@ -327,6 +327,7 @@ and 'loc tables = {
   remote_refs: 'loc remote_binding Remote_refs.builder;
   pattern_defs: 'loc parsed Pattern_defs.builder;
   patterns: 'loc pattern Patterns.builder;
+  mutable additional_errors: 'loc loc_node Signature_error.binding_validation_t list;
 }
 
 type frozen_kind =
@@ -344,6 +345,7 @@ let create_tables () =
     remote_refs = Remote_refs.create ();
     pattern_defs = Pattern_defs.create ();
     patterns = Patterns.create ();
+    additional_errors = [];
   }
 
 let push_loc tbls = Locs.push tbls.locs
@@ -1092,6 +1094,88 @@ module Scope = struct
   (* a `declare namespace` exports every binding. *)
   let finalize_declare_namespace_exn ~is_type_only scope tbls id_loc name =
     match scope with
+    (* Special declaration merging rules for namespaces in the global scope *)
+    | DeclareNamespace { values; types; parent = Global global_scope; _ } ->
+      let (values, types) = namespace_binding_of_values_and_types scope (values, types) in
+      let def : _ local_binding = NamespaceBinding { id_loc; name; values; types } in
+      let union_values_and_types existing_values existing_types values types =
+        let new_binding name (l, _) =
+          if SMap.mem name existing_values || SMap.mem name existing_types then (
+            tbls.additional_errors <- Signature_error.NameAlreadyBound l :: tbls.additional_errors;
+            false
+          ) else
+            true
+        in
+        let values = SMap.filter new_binding values in
+        let types = SMap.filter new_binding types in
+        (SMap.union existing_values values, SMap.union existing_types types)
+      in
+      (* When we decide that the namespace binding is mergeable. We call this function to
+       * mutate the existing node with merged names. *)
+      let merge_with_existing_local_binding_node (node : _ local_binding Local_defs.node) =
+        Local_defs.modify node (function
+            | NamespaceBinding { id_loc; name; values = existing_values; types = existing_types } ->
+              let (values, types) =
+                union_values_and_types existing_values existing_types values types
+              in
+              NamespaceBinding { id_loc; name; values; types }
+            | b -> b
+            )
+      in
+      (* This updater handle the simple case when we can directly update one kind of binding map
+       * (either value or type-only bindings). *)
+      let simple_updater = function
+        | Some (LocalBinding node) as existing_binding ->
+          merge_with_existing_local_binding_node node;
+          existing_binding
+        | Some (RemoteBinding _) as existing_binding -> existing_binding
+        | None ->
+          let node = push_local_def tbls def in
+          Some (LocalBinding node)
+      in
+      let updater values_map types_map =
+        match (is_type_only, SMap.find_opt name values_map, SMap.find_opt name types_map) with
+        | (_, Some _, Some _) ->
+          failwith "Invariant violation: a name cannot be in both values and types"
+        (* Simple case: no existing binding exist *)
+        | (false, None, None) -> (SMap.update name simple_updater values_map, types_map)
+        | (true, None, None) -> (values_map, SMap.update name simple_updater types_map)
+        (* Simple case: existing binding exist, but in the same value/type category *)
+        | (false, Some _, None) -> (SMap.update name simple_updater values_map, types_map)
+        | (true, None, Some _) -> (values_map, SMap.update name simple_updater types_map)
+        (* Originally not-type-only, now merging with a type-only namespace,
+         * so it's still not type-only *)
+        | (true, Some _, None) -> (SMap.update name simple_updater values_map, types_map)
+        (* Originally type-only, but now it's merging with a non-type-only namespace.
+         * The namespace binding needs to be promoted to a non-type-only namespace.
+         * (implemented by removing the node from type-only map, add to the value map, and
+         * mutate the binding node in place with additional names.)
+         *
+         * e.g. Consider
+         * ```
+         * declare namespace ns { type A = '' };
+         * declare namespace ns { declare const b: string };
+         * ```
+         *
+         * In order for the behavior to be equivalent to
+
+         * ```
+         * declare namespace ns { type A = ''; declare const b: string };
+         * ```
+         * We have to promote the original type-only namespace to a value binding.
+         *)
+        | (false, None, Some (LocalBinding node))
+          when match Local_defs.value node with
+               | NamespaceBinding _ -> true
+               | _ -> false ->
+          merge_with_existing_local_binding_node node;
+          (SMap.add name (LocalBinding node) values_map, SMap.remove name types_map)
+        (* Has an existing non-namespace binding, do nothing *)
+        | (false, None, Some _) -> (values_map, types_map)
+      in
+      let (values, types) = updater global_scope.values global_scope.types in
+      global_scope.values <- values;
+      global_scope.types <- types
     | DeclareNamespace { values; types; parent; _ } ->
       let (values, types) = namespace_binding_of_values_and_types scope (values, types) in
       bind_local
@@ -1100,6 +1184,7 @@ module Scope = struct
         tbls
         name
         (NamespaceBinding { id_loc; name; values; types })
+        ignore2
     | _ -> failwith "The scope must be lexical"
 
   let bind_globalThis scope tbls ~global_this_loc =
@@ -4172,7 +4257,7 @@ let namespace_decl
       comments = _;
     } =
   match id with
-  | Ast.Statement.DeclareNamespace.Global _ -> ignore
+  | Ast.Statement.DeclareNamespace.Global _ -> ()
   | Ast.Statement.DeclareNamespace.Local (id_loc, { Ast.Identifier.name; _ }) ->
     let id_loc = push_loc tbls id_loc in
     let stmts =
@@ -4677,7 +4762,7 @@ let rec statement opts scope tbls (loc, stmt) =
     Scope.finalize_declare_module_exports_exn scope
   | S.DeclareNamespace decl ->
     let is_type_only = Flow_ast_utils.is_type_only_declaration_statement (loc, stmt) in
-    namespace_decl opts scope tbls ~is_type_only ~visit_statement:statement decl ignore2
+    namespace_decl opts scope tbls ~is_type_only ~visit_statement:statement decl
   | S.DeclareEnum decl
   | S.EnumDeclaration decl ->
     enum_decl opts scope tbls decl ignore2
diff --git a/src/parser_utils/type_sig/type_sig_utils.ml b/src/parser_utils/type_sig/type_sig_utils.ml
index dfdc2cc5390..62872f75ad2 100644
--- a/src/parser_utils/type_sig/type_sig_utils.ml
+++ b/src/parser_utils/type_sig/type_sig_utils.ml
@@ -28,12 +28,31 @@ let parse_libs opts ordered_asts =
   Scope.bind_globalThis scope tbls ~global_this_loc:Loc.none;
   (tbls, Scope.builtins_exn scope)
 
+let create_pack_cx additional_errors =
+  Pack.create_cx
+    (List.map
+       (fun e ->
+         Type_sig.BindingValidationError (Signature_error.map_binding_validation_t Locs.index_exn e))
+       additional_errors
+    )
+
 let pack_builtins (tbls, (global_values, global_types, global_modules)) =
-  let { Parse.locs; module_refs; local_defs; remote_refs; pattern_defs; patterns } = tbls in
+  let {
+    Parse.locs;
+    module_refs;
+    local_defs;
+    remote_refs;
+    pattern_defs;
+    patterns;
+    additional_errors;
+  } =
+    tbls
+  in
   (* mark *)
   SMap.iter (fun _ b -> Mark.mark_binding ~locs_to_dirtify:[] b) global_values;
   SMap.iter (fun _ b -> Mark.mark_binding ~locs_to_dirtify:[] b) global_types;
   SMap.iter (fun _ m -> Mark.mark_builtin_module m) global_modules;
+  Mark.mark_errors additional_errors;
   (* compact *)
   let locs = Locs.compact locs in
   let module_refs = Module_refs.Interned.compact module_refs in
@@ -42,7 +61,7 @@ let pack_builtins (tbls, (global_values, global_types, global_modules)) =
   let pattern_defs = Pattern_defs.compact pattern_defs in
   let patterns = Patterns.compact patterns in
   (* copy *)
-  let cx = Pack.create_cx () in
+  let cx = create_pack_cx additional_errors in
   let (locs, _) = Locs.copy (fun x -> x) locs in
   let (module_refs, _) = Module_refs.copy (fun x -> x) module_refs in
   let (local_defs, _) = Local_defs.copy (Pack.pack_local_binding cx) local_defs in
@@ -97,9 +116,20 @@ let merge_locs loc0 loc1 =
       (Loc.debug_to_string ~include_source:true loc1)
 
 let pack ~locs_to_dirtify source (tbls, file_loc, exports) =
-  let { Parse.locs; module_refs; local_defs; remote_refs; pattern_defs; patterns } = tbls in
+  let {
+    Parse.locs;
+    module_refs;
+    local_defs;
+    remote_refs;
+    pattern_defs;
+    patterns;
+    additional_errors;
+  } =
+    tbls
+  in
   (* mark *)
   Mark.mark_exports ~locs_to_dirtify file_loc exports;
+  Mark.mark_errors additional_errors;
   (* compact *)
   let locs = Locs.compact ~merge:merge_locs locs in
   let module_refs = Module_refs.Interned.compact module_refs in
@@ -108,7 +138,7 @@ let pack ~locs_to_dirtify source (tbls, file_loc, exports) =
   let pattern_defs = Pattern_defs.compact pattern_defs in
   let patterns = Patterns.compact patterns in
   (* copy *)
-  let cx = Pack.create_cx () in
+  let cx = create_pack_cx additional_errors in
   let (locs, _) = Locs.copy (fun x -> x) locs in
   let (module_refs, _) = Module_refs.copy (fun x -> x) module_refs in
   let (local_defs, dirty_local_defs) = Local_defs.copy (Pack.pack_local_binding cx) local_defs in
diff --git a/src/parsing/parsing_service_js.ml b/src/parsing/parsing_service_js.ml
index ae73a528ed9..1d6e45a9e3c 100644
--- a/src/parsing/parsing_service_js.ml
+++ b/src/parsing/parsing_service_js.ml
@@ -211,6 +211,13 @@ let do_parse ~options ~docblock ?(locs_to_dirtify = []) content file =
                 | Type_sig.SigError err ->
                   let err = Signature_error.map (Type_sig_collections.Locs.get locs) err in
                   File_sig.SignatureVerificationError err :: acc
+                | Type_sig.BindingValidationError err ->
+                  let err =
+                    Signature_error.map_binding_validation_t
+                      (Type_sig_collections.Locs.get locs)
+                      err
+                  in
+                  File_sig.SignatureBindingValidationError err :: acc
                 | Type_sig.CheckError -> acc)
               []
               sig_errors
diff --git a/src/services/inference/inference_utils.ml b/src/services/inference/inference_utils.ml
index 5257ede1ab3..7f6bc3c41bd 100644
--- a/src/services/inference/inference_utils.ml
+++ b/src/services/inference/inference_utils.ml
@@ -53,8 +53,12 @@ let set_of_parse_exception ~source_file =
 let error_of_file_sig_tolerable_error ~source_file err =
   let open File_sig in
   let flow_err =
-    let (SignatureVerificationError sve) = err in
-    Error_message.ESignatureVerification (Signature_error.map ALoc.of_loc sve)
+    match err with
+    | SignatureVerificationError sve ->
+      Error_message.ESignatureVerification (Signature_error.map ALoc.of_loc sve)
+    | SignatureBindingValidationError sve ->
+      Error_message.ESignatureBindingValidation
+        (Signature_error.map_binding_validation_t ALoc.of_loc sve)
   in
   Flow_error.error_of_msg ~source_file flow_err
 
diff --git a/src/typing/debug_js.ml b/src/typing/debug_js.ml
index c87e628584b..a22123427b7 100644
--- a/src/typing/debug_js.ml
+++ b/src/typing/debug_js.ml
@@ -1636,6 +1636,8 @@ let dump_error_message =
     | EUnexpectedTemporaryBaseType loc ->
       spf "EUnexpectedTemporaryBaseType (%s)" (string_of_aloc loc)
     | ECannotDelete (l1, r1) -> spf "ECannotDelete (%s, %s)" (string_of_aloc l1) (dump_reason cx r1)
+    | ESignatureBindingValidation (Signature_error.NameAlreadyBound l) ->
+      spf "ESignatureBindingValidation (NameAlreadyBound %s)" (string_of_aloc l)
     | ESignatureVerification sve ->
       let msg = string_of_signature_error ALoc.debug_to_string sve in
       spf "ESignatureVerification (%s)" msg
diff --git a/src/typing/errors/error_message.ml b/src/typing/errors/error_message.ml
index 43d52eef0dd..45389d10f4b 100644
--- a/src/typing/errors/error_message.ml
+++ b/src/typing/errors/error_message.ml
@@ -378,6 +378,7 @@ and 'loc t' =
   | EUnnecessaryDeclareTypeOnlyExport of 'loc
   | EUnexpectedTemporaryBaseType of 'loc
   | ECannotDelete of 'loc * 'loc virtual_reason
+  | ESignatureBindingValidation of 'loc Signature_error.binding_validation_t
   | ESignatureVerification of 'loc Signature_error.t
   | EPrimitiveAsInterface of {
       use_op: 'loc virtual_use_op;
@@ -1195,6 +1196,8 @@ let rec map_loc_of_error_message (f : 'a -> 'b) : 'a t' -> 'b t' =
   | EUnnecessaryDeclareTypeOnlyExport loc -> EUnnecessaryDeclareTypeOnlyExport (f loc)
   | EUnexpectedTemporaryBaseType loc -> EUnexpectedTemporaryBaseType (f loc)
   | ECannotDelete (l1, r1) -> ECannotDelete (f l1, map_reason r1)
+  | ESignatureBindingValidation sve ->
+    ESignatureBindingValidation (Signature_error.map_binding_validation_t f sve)
   | ESignatureVerification sve -> ESignatureVerification (Signature_error.map f sve)
   | EPrimitiveAsInterface { use_op; reason; interface_reason; kind } ->
     EPrimitiveAsInterface
@@ -1713,6 +1716,7 @@ let util_use_op_of_msg nope util = function
   | EUnnecessaryDeclareTypeOnlyExport _
   | EUnexpectedTemporaryBaseType _
   | ECannotDelete _
+  | ESignatureBindingValidation _
   | ESignatureVerification _
   | EExponentialSpread _
   | EComputedPropertyWithUnion _
@@ -1967,6 +1971,11 @@ let loc_of_msg : 'loc t' -> 'loc option = function
   | EMissingTypeArgs { reason_op; _ } -> Some (loc_of_reason reason_op)
   | EInvalidRendersTypeArgument { loc; _ } -> Some loc
   | EInvalidTypeCastSyntax { loc; _ } -> Some loc
+  | ESignatureBindingValidation e ->
+    Signature_error.(
+      (match e with
+      | NameAlreadyBound loc -> Some loc)
+    )
   | ESignatureVerification sve ->
     Signature_error.(
       (match sve with
@@ -2616,6 +2625,8 @@ let friendly_message_of_msg = function
     Normal (MessageCannotExportRenamedDefault { name; is_reexport })
   | EUnexpectedTemporaryBaseType _ -> Normal MessageUnexpectedTemporaryBaseType
   | ECannotDelete (_, expr) -> Normal (MessageCannotDelete expr)
+  | ESignatureBindingValidation (Signature_error.NameAlreadyBound _) ->
+    Normal MessageCannotDeclareAlreadyBoundNameInLibdef
   | ESignatureVerification sve -> Normal (MessageCannotBuildTypedInterface sve)
   | EUnreachable _ -> Normal MessageUnreachableCode
   | EInvalidObjectKit { reason; reason_op = _; use_op } ->
@@ -3223,6 +3234,7 @@ let error_code_of_message err : error_code option =
   (* We don't want these to be suppressible *)
   | ERecursionLimit (_, _) -> None
   | EROArrayWrite (_, use_op) -> react_rule_of_use_op use_op ~default:CannotWrite
+  | ESignatureBindingValidation _ -> Some SignatureVerificationFailure
   | ESignatureVerification _ -> Some SignatureVerificationFailure
   | EThisInExportedFunction _ -> Some ThisInExportedFunction
   | EExportRenamedDefault _ -> Some ExportRenamedDefault
diff --git a/src/typing/errors/flow_intermediate_error.ml b/src/typing/errors/flow_intermediate_error.ml
index fc6618c1ec6..c035f54e303 100644
--- a/src/typing/errors/flow_intermediate_error.ml
+++ b/src/typing/errors/flow_intermediate_error.ml
@@ -1637,6 +1637,8 @@ let to_printable_error :
       ]
     | MessageCannotDeclareAlreadyBoundName x ->
       [text "Cannot declare "; Friendly.ref x; text " because the name is already bound."]
+    | MessageCannotDeclareAlreadyBoundNameInLibdef ->
+      [text "Cannot declare the name in library definition because the name is already bound."]
     | MessageCannotDelete expr ->
       [
         text "Cannot delete ";
diff --git a/src/typing/errors/flow_intermediate_error_types.ml b/src/typing/errors/flow_intermediate_error_types.ml
index fe833c9f5bb..ad98ac1ddd1 100644
--- a/src/typing/errors/flow_intermediate_error_types.ml
+++ b/src/typing/errors/flow_intermediate_error_types.ml
@@ -396,6 +396,7 @@ type 'loc message =
   | MessageCannotCreateExactType of 'loc virtual_reason
   | MessageCannotDeclareAlreadyBoundGlobal of concrete_reason
   | MessageCannotDeclareAlreadyBoundName of concrete_reason
+  | MessageCannotDeclareAlreadyBoundNameInLibdef
   | MessageCannotDelete of 'loc virtual_reason
   | MessageCannotDetermineEmptyArrayLiteralType
   | MessageCannotDetermineModuleType
diff --git a/src/typing/merge_js.ml b/src/typing/merge_js.ml
index d36938eb76d..c53305507a8 100644
--- a/src/typing/merge_js.ml
+++ b/src/typing/merge_js.ml
@@ -884,6 +884,18 @@ let merge_lib_files ~sig_opts ordered_asts =
                  (msg |> Error_message.loc_of_msg |> Base.Option.bind ~f:ALoc.source)
              in
              Some (Flow_error.error_of_msg ~source_file msg)
+           | Type_sig.BindingValidationError e ->
+             let e =
+               Signature_error.map_binding_validation_t
+                 (fun l -> l |> Type_sig_collections.Locs.get builtin_locs |> ALoc.of_loc)
+                 e
+             in
+             let msg = Error_message.ESignatureBindingValidation e in
+             let source_file =
+               Base.Option.value_exn
+                 (msg |> Error_message.loc_of_msg |> Base.Option.bind ~f:ALoc.source)
+             in
+             Some (Flow_error.error_of_msg ~source_file msg)
            | Type_sig.CheckError -> None
            )
     |> Flow_error.ErrorSet.of_list
diff --git a/tests/declare_namespace/declare_namespace.exp b/tests/declare_namespace/declare_namespace.exp
index 7bff5f83409..251a66ba4db 100644
--- a/tests/declare_namespace/declare_namespace.exp
+++ b/tests/declare_namespace/declare_namespace.exp
@@ -1,3 +1,19 @@
+Error ------------------------------------------------------------------------------------- flow-typed/namespaces.js:8:8
+
+Cannot declare the name in library definition because the name is already bound. [signature-verification-failure]
+
+   8|   type a = string; // error: already bound
+             ^
+
+
+Error ------------------------------------------------------------------------------------- flow-typed/namespaces.js:9:8
+
+Cannot declare the name in library definition because the name is already bound. [signature-verification-failure]
+
+   9|   type b = string; // error: already bound
+             ^
+
+
 Error ---------------------------------------------------------------------------------------------- binding_test.js:2:6
 
 Cannot declare `conflict_with_type_decl_1` [1] because the name is already bound. [name-already-bound]
@@ -331,6 +347,120 @@ References:
                          ^^^^^^^^^^^ [1]
 
 
+Error ----------------------------------------------------------------------------------------------- libdef_test.js:1:1
+
+Cannot cast `ns_v.a` to empty because string [1] is incompatible with empty [2]. [incompatible-cast]
+
+   libdef_test.js:1:1
+   1| ns_v.a as empty; // error: string ~> empty
+      ^^^^^^
+
+References:
+   flow-typed/namespaces.js:2:20
+   2|   declare const a: string;
+                         ^^^^^^ [1]
+   libdef_test.js:1:11
+   1| ns_v.a as empty; // error: string ~> empty
+                ^^^^^ [2]
+
+
+Error ----------------------------------------------------------------------------------------------- libdef_test.js:2:1
+
+Cannot cast `ns_v.b` to empty because string [1] is incompatible with empty [2]. [incompatible-cast]
+
+   libdef_test.js:2:1
+   2| ns_v.b as empty; // error: string ~> empty
+      ^^^^^^
+
+References:
+   flow-typed/namespaces.js:5:20
+   5|   declare const b: string;
+                         ^^^^^^ [1]
+   libdef_test.js:2:11
+   2| ns_v.b as empty; // error: string ~> empty
+                ^^^^^ [2]
+
+
+Error ----------------------------------------------------------------------------------------------- libdef_test.js:3:1
+
+Cannot resolve name `ns_t`. [cannot-resolve-name]
+
+   3| ns_t; // error: it's a type only namespace
+      ^^^^
+
+
+Error ----------------------------------------------------------------------------------------------- libdef_test.js:4:1
+
+Cannot cast `1` to `T1` because number [1] is incompatible with string [2]. [incompatible-cast]
+
+   libdef_test.js:4:1
+   4| 1 as ns_t.T1; // error: number ~> string
+      ^ [1]
+
+References:
+   libdef_test.js:4:6
+   4| 1 as ns_t.T1; // error: number ~> string
+           ^^^^^^^ [2]
+
+
+Error ----------------------------------------------------------------------------------------------- libdef_test.js:5:1
+
+Cannot cast `ns_v_and_then_t` to empty because namespace ns_v_and_then_t [1] is incompatible with empty [2].
+[incompatible-cast]
+
+   libdef_test.js:5:1
+    5| ns_v_and_then_t as empty; // error: value namespace ~> empty
+       ^^^^^^^^^^^^^^^
+
+References:
+   flow-typed/namespaces.js:17:19
+   17| declare namespace ns_v_and_then_t {
+                         ^^^^^^^^^^^^^^^ [1]
+   libdef_test.js:5:20
+    5| ns_v_and_then_t as empty; // error: value namespace ~> empty
+                          ^^^^^ [2]
+
+
+Error ----------------------------------------------------------------------------------------------- libdef_test.js:6:1
+
+Cannot cast `ns_t_and_then_v` to empty because namespace ns_t_and_then_v [1] is incompatible with empty [2].
+[incompatible-cast]
+
+   libdef_test.js:6:1
+    6| ns_t_and_then_v as empty; // error: value namespace ~> empty
+       ^^^^^^^^^^^^^^^
+
+References:
+   flow-typed/namespaces.js:20:19
+   20| declare namespace ns_t_and_then_v {
+                         ^^^^^^^^^^^^^^^ [1]
+   libdef_test.js:6:20
+    6| ns_t_and_then_v as empty; // error: value namespace ~> empty
+                          ^^^^^ [2]
+
+
+Error ---------------------------------------------------------------------------------------------- libdef_test.js:8:14
+
+Cannot get `non_ns_value.b` because property `b` is missing in `String` [1]. [prop-missing]
+
+   libdef_test.js:8:14
+    8| non_ns_value.b as empty; // error: value namespace ~> empty
+                    ^
+
+References:
+   flow-typed/namespaces.js:30:29
+   30| declare const non_ns_value: string;
+                                   ^^^^^^ [1]
+
+
+Error ----------------------------------------------------------------------------------------------- libdef_test.js:9:1
+
+Cannot resolve name `non_ns_type`. [cannot-resolve-name]
+
+   9| non_ns_type; // error: it's a type
+      ^^^^^^^^^^^
+
+
 Error --------------------------------------------------------------------------------- type_only_react_namespace.js:3:3
 
 Cannot resolve name `React`. [cannot-resolve-name]
@@ -777,7 +907,7 @@ References:
 
 
 
-Found 56 errors
+Found 66 errors
 
 Only showing the most relevant union/intersection branches.
 To see all branches, re-run Flow with --show-all-branches
diff --git a/tests/declare_namespace/flow-typed/namespaces.js b/tests/declare_namespace/flow-typed/namespaces.js
new file mode 100644
index 00000000000..ef477449c7e
--- /dev/null
+++ b/tests/declare_namespace/flow-typed/namespaces.js
@@ -0,0 +1,44 @@
+declare namespace ns_v {
+  declare const a: string;
+}
+declare namespace ns_v {
+  declare const b: string;
+}
+declare namespace ns_v {
+  type a = string; // error: already bound
+  type b = string; // error: already bound
+}
+declare namespace ns_t {
+  type T1 = string;
+}
+declare namespace ns_t {
+  type T2 = string;
+}
+declare namespace ns_v_and_then_t {
+  declare const a: string;
+}
+declare namespace ns_t_and_then_v {
+  type T1 = string;
+}
+declare namespace ns_v_and_then_t {
+  type T1 = string;
+}
+declare namespace ns_t_and_then_v {
+  declare const a: string;
+}
+
+declare const non_ns_value: string;
+type non_ns_type = string;
+// The following namespaces won't have any effect
+declare namespace non_ns_value {
+  declare const b: string;
+}
+declare namespace non_ns_value {
+  type T1 = string;
+}
+declare namespace non_ns_type {
+  declare const b: string;
+}
+declare namespace non_ns_type {
+  type T1 = string;
+}
diff --git a/tests/declare_namespace/libdef_test.js b/tests/declare_namespace/libdef_test.js
new file mode 100644
index 00000000000..676445fb939
--- /dev/null
+++ b/tests/declare_namespace/libdef_test.js
@@ -0,0 +1,10 @@
+ns_v.a as empty; // error: string ~> empty
+ns_v.b as empty; // error: string ~> empty
+ns_t; // error: it's a type only namespace
+1 as ns_t.T1; // error: number ~> string
+ns_v_and_then_t as empty; // error: value namespace ~> empty
+ns_t_and_then_v as empty; // error: value namespace ~> empty
+
+non_ns_value.b as empty; // error: value namespace ~> empty
+non_ns_type; // error: it's a type
+type _t2 = non_ns_type; // ok