diff --git a/packages/api/src/core/cors.test.ts b/packages/api/src/core/cors.test.ts
index 3bca97c..505ec31 100644
--- a/packages/api/src/core/cors.test.ts
+++ b/packages/api/src/core/cors.test.ts
@@ -97,5 +97,48 @@ describe("ENG-1666 CORS conformance", () => {
       );
       expect(res.headers.get("Access-Control-Allow-Origin")).toBeNull();
     });
+
+    it("overwrites a stale Allow-Origin with the correct value (no duplicates)", () => {
+      process.env.ALLOWED_ORIGINS = "https://app.example.com";
+      const upstream = new Response(JSON.stringify({ ok: true }), {
+        headers: {
+          "Content-Type": "application/json",
+          "Access-Control-Allow-Origin": "https://stale.example.com",
+        },
+      });
+      const res = addCorsHeaders(upstream, makeReq("https://app.example.com"));
+      // Only the correct value, emitted exactly once.
+      expect(res.headers.get("Access-Control-Allow-Origin")).toBe(
+        "https://app.example.com",
+      );
+    });
+
+    it("strips a stale Allow-Origin when the origin is not allowed", () => {
+      process.env.ALLOWED_ORIGINS = "https://app.example.com";
+      const upstream = new Response(JSON.stringify({ ok: true }), {
+        headers: {
+          "Content-Type": "application/json",
+          "Access-Control-Allow-Origin": "https://stale.example.com",
+        },
+      });
+      const res = addCorsHeaders(upstream, makeReq("https://evil.example.com"));
+      expect(res.headers.get("Access-Control-Allow-Origin")).toBeNull();
+    });
+
+    it("strips a stale Allow-Credentials header", () => {
+      process.env.ALLOWED_ORIGINS = "https://app.example.com";
+      const upstream = new Response(JSON.stringify({ ok: true }), {
+        headers: {
+          "Content-Type": "application/json",
+          "Access-Control-Allow-Origin": "https://stale.example.com",
+          "Access-Control-Allow-Credentials": "true",
+        },
+      });
+      const res = addCorsHeaders(upstream, makeReq("https://evil.example.com"));
+      expect(res.headers.get("Access-Control-Allow-Origin")).toBeNull();
+      expect(
+        res.headers.get("Access-Control-Allow-Credentials"),
+      ).toBeNull();
+    });
   });
 });
diff --git a/packages/api/src/core/cors.ts b/packages/api/src/core/cors.ts
index ba69913..6ebd0e4 100644
--- a/packages/api/src/core/cors.ts
+++ b/packages/api/src/core/cors.ts
@@ -34,7 +34,14 @@ export function corsHeadersFor(req: Request): Record<string, string> {
 
 export function addCorsHeaders(response: Response, req: Request): Response {
   const newHeaders = new Headers(response.headers);
-  for (const [key, value] of Object.entries(corsHeadersFor(req))) {
+  // Remove any pre-existing/stale CORS headers so an upstream
+  // Access-Control-Allow-Origin can never leak or be emitted twice.
+  // corsHeadersFor is re-applied below with the values for the evaluated origin;
+  // when the origin is not allowed, ACAO stays absent (secure by default).
+  newHeaders.delete("Access-Control-Allow-Origin");
+  newHeaders.delete("Access-Control-Allow-Credentials");
+  const corsHeaders = corsHeadersFor(req);
+  for (const [key, value] of Object.entries(corsHeaders)) {
     newHeaders.set(key, value);
   }
   return new Response(response.body, {