Warn on duplicate #suppress directives on the same node

.chronus/changes/copilot-add-warning-on-double-suppression-2026-5-30-14-49-17.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Warn on duplicate `#suppress` directives on the same node.

packages/compiler/src/core/messages.ts

@@ -746,6 +746,12 @@ const diagnostics = {
       default: paramMessage`Duplicate name: "${"name"}"`,
     },
   },
+  "duplicate-suppression": {
+    severity: "warning",
+    messages: {
+      default: paramMessage`Diagnostic "${"code"}" is already suppressed on this node.`,
+    },
+  },
   "decorator-decl-target": {
     severity: "error",
     messages: {

packages/compiler/src/core/program.ts

@@ -45,6 +45,7 @@ import {
   SuppressionTracker,
   createSuppressionTracker,
   findDirectiveSuppressingOnNode,
+  findDuplicateSuppressions,
 } from "./suppression-tracking.js";
 import {
   CompilerHost,
@@ -305,6 +306,15 @@ async function createProgram(
   oldProgram = undefined;
 
   suppressionTracker = createSuppressionTracker(sourceResolution);
+  reportDiagnostics(
+    findDuplicateSuppressions(sourceResolution).map(({ directive }) =>
+      createDiagnostic({
+        code: "duplicate-suppression",
+        format: { code: directive.code },
+        target: directive.node,
+      }),
+    ),
+  );
 
   const resolver = (program.resolver = createResolver(program));
   runtimeStats.resolver = perf.time(() => resolver.resolveProgram());

packages/compiler/src/core/suppression-tracking.ts

@@ -16,6 +16,10 @@ export interface UnusedSuppression {
   directive: SuppressDirective;
 }
 
+export interface DuplicateSuppression {
+  directive: SuppressDirective;
+}
+
 export interface SuppressionTracker {
   markUsed(directiveNode: DirectiveExpressionNode): void;
   getUnusedSuppressions(): UnusedSuppression[];
@@ -53,6 +57,40 @@ export function createSuppressionTracker(sourceResolution: SourceResolution): Su
   };
 }
 
+export function findDuplicateSuppressions(
+  sourceResolution: SourceResolution,
+): DuplicateSuppression[] {
+  const duplicates: DuplicateSuppression[] = [];
+  for (const script of sourceResolution.sourceFiles.values()) {
+    if (sourceResolution.locationContexts.get(script.file)?.type !== "project") {
+      continue;
+    }
+
+    visit(script);
+  }
+
+  return duplicates;
+
+  function visit(node: Node) {
+    const seenSuppressions = new Set<string>();
+    for (const directiveNode of node.directives ?? []) {
+      const directive = parseDirective(directiveNode);
+      if (directive?.name !== "suppress") {
+        continue;
+      }
+
+      if (seenSuppressions.has(directive.code)) {
+        duplicates.push({ directive });
+        continue;
+      }
+
+      seenSuppressions.add(directive.code);
+    }
+
+    visitChildren(node, visit);
+  }
+}
+
 interface SuppressionRecord {
   directive: SuppressDirective;
   used: boolean;

packages/compiler/test/suppression.test.ts

@@ -86,6 +86,54 @@ it("error diagnostics cannot be suppressed and emit another error", async () =>
   expectDiagnostics(diagnostics, [{ code: "suppress-error" }, { code: "no-id-property" }]);
 });
 
+it("warns on duplicate suppressions with a message", async () => {
+  const [, diagnostics] = await Tester.compileAndDiagnose(
+    `
+      #deprecated "Old is deprecated"
+      model Old {}
+
+      model Foo {
+        #suppress "deprecated" "intentional"
+        #suppress "deprecated" "duplicate"
+        prop: Old;
+      }
+      `,
+    {
+      compilerOptions: { nostdlib: true },
+    },
+  );
+
+  expectDiagnostics(diagnostics, {
+    code: "duplicate-suppression",
+    severity: "warning",
+    message: 'Diagnostic "deprecated" is already suppressed on this node.',
+  });
+});
+
+it("warns on duplicate suppressions without a message", async () => {
+  const [, diagnostics] = await Tester.compileAndDiagnose(
+    `
+      #deprecated "Old is deprecated"
+      model Old {}
+
+      model Foo {
+        #suppress "deprecated"
+        #suppress "deprecated"
+        prop: Old;
+      }
+      `,
+    {
+      compilerOptions: { nostdlib: true },
+    },
+  );
+
+  expectDiagnostics(diagnostics, {
+    code: "duplicate-suppression",
+    severity: "warning",
+    message: 'Diagnostic "deprecated" is already suppressed on this node.',
+  });
+});
+
 it("reports unused suppression via tracker", async () => {
   const { program } = await Tester.compile(
     `