fix: append server-timing header value if key exist when using setHeaders to avoid exception (#14700)

Closes #14681

Author: tolu

.changeset/clear-spies-wear.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: allow multiple `server-timing` headers

packages/kit/src/runtime/server/respond.js

@@ -183,7 +183,12 @@ export async function internal_respond(request, options, manifest, state) {
 						'Use `event.cookies.set(name, value, options)` instead of `event.setHeaders` to set cookies'
 					);
 				} else if (lower in headers) {
-					throw new Error(`"${key}" header is already set`);
+					// appendHeaders-style for Server-Timing https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Server-Timing
+					if (lower === 'server-timing') {
+						headers[lower] += ', ' + value;
+					} else {
+						throw new Error(`"${key}" header is already set`);
+					}
 				} else {
 					headers[lower] = value;