Prevent duplicate Server-Timing header entries in Cloudflare

Author: onurtemizkan

packages/remix/src/server/serverTimingTracePropagation.ts

@@ -63,6 +63,11 @@ export function injectServerTimingHeaderValue(response: Response, serverTimingVa
     const headers = new Headers(response.headers);
     const existing = headers.get('Server-Timing');
 
+    // Avoid duplicate entries when manually injected in entry.server.tsx
+    if (existing?.includes('sentry-trace')) {
+      return response;
+    }
+
     headers.set('Server-Timing', existing ? `${existing}, ${serverTimingValue}` : serverTimingValue);
 
     return new Response(response.body, {

packages/remix/test/server/serverTimingTracePropagation.test.ts

@@ -127,5 +127,23 @@ describe('serverTimingTracePropagation', () => {
 
       expect(result.headers.get('Server-Timing')).toBe('cache;dur=100, sentry-trace;desc="test"');
     });
+
+    it('skips injection when sentry-trace already exists in Server-Timing header', () => {
+      const mockResponse = new Response('test body', {
+        status: 200,
+        headers: new Headers({
+          'Server-Timing': 'sentry-trace;desc="existing-trace", baggage;desc="existing-baggage"',
+        }),
+      });
+
+      const result = injectServerTimingHeaderValue(
+        mockResponse,
+        'sentry-trace;desc="new-trace", baggage;desc="new-baggage"',
+      );
+
+      expect(result.headers.get('Server-Timing')).toBe(
+        'sentry-trace;desc="existing-trace", baggage;desc="existing-baggage"',
+      );
+    });
   });
 });