Fix Dagger multibinding duplicate tracking for Maps

Currently, Dagger only verifies duplicate map key contributions within the same component. Contributions with duplicate keys across the component hierarchy are ignored, resulting in masked values or runtime crashes when backed by Guava.

This change introduces strict duplicate detection across the full map binding graph. Dagger now performs compile-time verification to reject duplicate map keys across the entire component hierarchy.

This behavior is controlled by the new compiler option -Adagger.mapMultibindingDuplicateDetectionFix. When enabled, strict duplicate tracking is enforced across the component hierarchy.

RELNOTES=Fixed Dagger multibinding duplicate masking across Map components (compile-time validation).
PiperOrigin-RevId: 915631609

Author: garabais

dev-guide/compiler-options.md

@@ -170,6 +170,23 @@ especially true when dealing with Kotlin sources, where wildcards must be
 interpreted indirectly from a number of factors other than the explicit variance
 at the use site.
 
+## Detect duplicate map multibinding contributions across component boundaries {#map-multibinding-duplicate-detection-fix}
+
+The `dagger.mapMultibindingDuplicateDetectionFix` flag fixes a bug in Dagger
+where duplicate multibinding map keys were allowed in certain situations.
+When the flag is enabled Dagger should enforce strict duplicate checking of map
+multibinding contributions (e.g., identical `@IntoMap` bindings) across
+component boundaries.
+
+By default, this flag is disabled, meaning Dagger preserves the legacy lenient
+behavior and ignores duplicate map key contributions across component
+boundaries.
+
+To opt-in to strict duplicate detection at compile time (for Maps), enable the
+flag by passing the following compiler option:
+
+`-Adagger.mapMultibindingDuplicateDetectionFix=ENABLED`
+
 <!-- References -->
 
 [`@Component`]: https://dagger.dev/api/latest/dagger/Component.html