Fix duplicate entity tracking error when re-attaching detached entities with client-generated keys

src/EFCore/ChangeTracking/Internal/InternalEntityEntry.cs

@@ -204,6 +204,18 @@ protected override void OnStateChanging(EntityState newState)
 
         if (EntityState == EntityState.Detached)
         {
+            // Check if this entity is already being tracked by a different entry.
+            // This can happen when the entity was detached and then re-tracked through
+            // navigation fixup (via DetectChanges) before the user manually sets its state.
+            var existingEntry = StateManager.TryGetEntry(Entity, EntityType);
+            if (existingEntry != null && existingEntry != this)
+            {
+                // The entity is already tracked by a different entry.
+                // Update the state of the existing entry instead of trying to track this stale entry.
+                existingEntry.SetEntityState(newState);
+                return;
+            }
+
             StateManager.StartTracking(this);
         }
     }

test/EFCore.Tests/ChangeTracking/GraphTrackingTest.cs

@@ -384,6 +384,70 @@ public void Can_add_two_aggregates_linked_down_the_tree()
         Assert.Equal(EntityState.Added, context.Entry(reminders[1]).State);
     }
 
+    [ConditionalFact]
+    public void Can_reattach_graph_with_client_generated_keys()
+    {
+        using var context = new GuidKeyContext();
+        var foo = new FooGuid { Name = "Foo1" };
+        var bar1 = new BarGuid { Name = "Bar1", Foo = foo };
+        var bar2 = new BarGuid { Name = "Bar2", Foo = foo, RelatedBar = bar1 };
+
+        var fooEntry = context.Foos.Add(foo);
+        var bar1Entry = context.Bars.Add(bar1);
+        var bar2Entry = context.Bars.Add(bar2);
+
+        fooEntry.State = EntityState.Detached;
+        bar1Entry.State = EntityState.Detached;
+        bar2Entry.State = EntityState.Detached;
+
+        // When detaching entities in Added state, EF Core automatically nulls out navigations
+        // to maintain consistency (since Added entities don't exist in the database yet).
+        // This means bar2.RelatedBar is now null and needs to be manually restored.
+
+        fooEntry.State = EntityState.Added;
+        fooEntry.DetectChanges();
+
+        bar2Entry.State = EntityState.Added;
+        bar2.RelatedBar = bar1; // Manually restore the navigation
+        bar2Entry.DetectChanges();
+
+        // This should not throw
+        bar1Entry.State = EntityState.Added;
+        bar1Entry.DetectChanges();
+
+        Assert.Equal(EntityState.Added, context.Entry(foo).State);
+        Assert.Equal(EntityState.Added, context.Entry(bar1).State);
+        Assert.Equal(EntityState.Added, context.Entry(bar2).State);
+    }
+
+    private class GuidKeyContext : DbContext
+    {
+        protected internal override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+            => optionsBuilder
+                .UseInMemoryDatabase(nameof(GuidKeyContext))
+                .UseInternalServiceProvider(InMemoryFixture.DefaultServiceProvider);
+
+        public DbSet<FooGuid> Foos => Set<FooGuid>();
+        public DbSet<BarGuid> Bars => Set<BarGuid>();
+    }
+
+    private class FooGuid
+    {
+        public Guid Id { get; set; }
+        public string Name { get; set; }
+        public ICollection<BarGuid> Bars { get; set; } = new HashSet<BarGuid>();
+    }
+
+    private class BarGuid
+    {
+        public Guid Id { get; set; }
+        public string Name { get; set; }
+        public FooGuid Foo { get; set; }
+        public Guid FooId { get; set; }
+        public BarGuid RelatedBar { get; set; }
+        public Guid? RelatedBarId { get; set; }
+    }
+
     private class AggregateContext : DbContext
     {
         protected internal override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)