Fix duplicate member entries in Scaladoc generation

Resolves an issue where Scaladoc was generating duplicate entries for
the same members when combining results from multiple parser instances.

The fix implements deduplication by creating a stable composite key from
the member's location, full name, and kind. When duplicates are found,
the entry with documentation is preferred, falling back to the first
entry if none have docs.

This ensures clean, non-redundant API documentation output.

Co-authored-by: Bill Venners <bill@artima.com>

Author: cheeseng

scaladoc/src/dotty/tools/scaladoc/tasty/TastyParser.scala

@@ -169,7 +169,12 @@ object ScaladocTastyInspector:
         val withNewMembers = p1.withNewMembers(p2.members)
         if withNewMembers.docs.isEmpty then withNewMembers.withDocs(p2.docs) else withNewMembers
       )
-      basePck.withMembers((basePck.members ++ rest).sortBy(_.name))
+      // Deduplicate members coming from different parser instances by a stable key:
+      // (location, member fullName, kind name).
+      val combined = basePck.members ++ rest
+      val keyed = combined.groupBy(m => (m.dri.location, m.fullName, m.kind.name))
+      val uniqueMembers = keyed.values.map(g => g.find(_.docs.nonEmpty).getOrElse(g.head)).toList.sortBy(_.name)
+      basePck.withMembers(uniqueMembers)
     }.toList -> inspector.rootDoc
 
 end ScaladocTastyInspector