From a6c452433675f023e90c28432a7379eb2bea9100 Mon Sep 17 00:00:00 2001
From: Will Temple <witemple@microsoft.com>
Date: Tue, 7 Oct 2025 15:20:52 -0400
Subject: [PATCH 1/3] core: exclude false members from @encodedName duplication
 tracking

---
 packages/compiler/src/lib/encoded-names.ts | 11 ++++++++++-
 packages/compiler/src/lib/visibility.ts    |  2 +-
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/packages/compiler/src/lib/encoded-names.ts b/packages/compiler/src/lib/encoded-names.ts
index 3ccb45ecdd2..a7c04b792f9 100644
--- a/packages/compiler/src/lib/encoded-names.ts
+++ b/packages/compiler/src/lib/encoded-names.ts
@@ -106,7 +106,16 @@ export function validateEncodedNamesConflicts(program: Program) {
 
   for (const [target, map] of getEncodedNamesStateMap(program).entries()) {
     const scope = getScope(target);
-    if (scope === undefined) {
+    const memberValues = new Set(scope?.members.values());
+    if (
+      scope === undefined ||
+      // Workaround: exclude members that aren't actually in the scope. This can happen when types of
+      // properties, enum members, or union variants are cloned for one reason or another, but are not
+      // actually attached to the parent type. This should probably be fixed in cloning logic (mutator
+      // implementation) instead, but for now this workaround solves some problems with false positives
+      // in duplication tracking here.
+      !memberValues.has(target)
+    ) {
       return;
     }
     for (const [mimeType, name] of map.entries()) {
diff --git a/packages/compiler/src/lib/visibility.ts b/packages/compiler/src/lib/visibility.ts
index b746dd3b4e2..591919c77d9 100644
--- a/packages/compiler/src/lib/visibility.ts
+++ b/packages/compiler/src/lib/visibility.ts
@@ -681,7 +681,7 @@ function createVisibilityFilterMutator(
             realm.remove(clone);
             modified = true;
           } else {
-            const mutated = mutateSubgraph(program, [mpMutator], prop);
+            const mutated = cachedMutateSubgraph(program, mpMutator, prop);
 
             clone.properties.set(key, mutated.type as ModelProperty);
 

From fb0002c8a6ae1ccc17d3cde6f9380455101aa26f Mon Sep 17 00:00:00 2001
From: Will Temple <witemple@microsoft.com>
Date: Tue, 7 Oct 2025 15:21:45 -0400
Subject: [PATCH 2/3] chronus

---
 ...le-msft-fix-encodedname-dup-detect-2025-9-7-15-21-36.md | 7 +++++++
 1 file changed, 7 insertions(+)
 create mode 100644 .chronus/changes/witemple-msft-fix-encodedname-dup-detect-2025-9-7-15-21-36.md

diff --git a/.chronus/changes/witemple-msft-fix-encodedname-dup-detect-2025-9-7-15-21-36.md b/.chronus/changes/witemple-msft-fix-encodedname-dup-detect-2025-9-7-15-21-36.md
new file mode 100644
index 00000000000..88a93e23eb9
--- /dev/null
+++ b/.chronus/changes/witemple-msft-fix-encodedname-dup-detect-2025-9-7-15-21-36.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Addressed a bug that could cause duplicate `@encodedName` applications to be detected when none actually exist.
\ No newline at end of file

From 18ce33d4b5c6beccd563cc2f1d6f57f681633639 Mon Sep 17 00:00:00 2001
From: Will Temple <witemple@microsoft.com>
Date: Tue, 28 Oct 2025 10:50:53 -0400
Subject: [PATCH 3/3] test

---
 packages/compiler/test/visibility.test.ts | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/packages/compiler/test/visibility.test.ts b/packages/compiler/test/visibility.test.ts
index 82a2d892dfd..5247547e08f 100644
--- a/packages/compiler/test/visibility.test.ts
+++ b/packages/compiler/test/visibility.test.ts
@@ -1239,6 +1239,20 @@ describe("compiler: visibility core", () => {
     ok(arrA.properties.has("a"));
     ok(!arrA.properties.has("invisible"));
   });
+
+  it("does not duplicate encodedName metadata", async () => {
+    const diagnostics = await runner.diagnose(`
+      model SomeModel {
+        @visibility(Lifecycle.Read)
+        @encodedName("application/json", "some_other_name")
+        someOtherName: string;
+      }
+
+      alias ReadModel = Read<SomeModel>;
+    `);
+
+    expectDiagnosticEmpty(diagnostics);
+  });
 });
 
 function validateCreateOrUpdateTransform(