Don't override cache-control if it has already been set

index.js

@@ -75,7 +75,9 @@ function fastifyCaching (instance, options, next) {
     }
 
     instance.addHook('onRequest', (req, res, next) => {
-      res.header('Cache-control', value)
+      if (!res.hasHeader('Cache-control')) {
+        res.header('Cache-control', value)
+      }
       next()
     })
   }

test/headers.test.js

@@ -139,6 +139,35 @@ test('sets public with max-age and s-maxage header', async (t) => {
   t.equal(response.headers['cache-control'], 'public, max-age=300, s-maxage=12345')
 })
 
+test('do not set headers if another upstream plugin already sets it', async (t) => {
+  t.plan(2)
+
+  const opts = {
+    privacy: plugin.privacy.PUBLIC,
+    expiresIn: 300,
+    serverExpiresIn: 12345
+  }
+
+  const fastify = Fastify()
+  fastify.addHook('onRequest', async (req, reply) => {
+    reply.header('cache-control', 'do not override')
+  })
+  await fastify.register(plugin, opts)
+
+  fastify.get('/', (req, reply) => {
+    reply.send({ hello: 'world' })
+  })
+
+  await fastify.ready()
+
+  const response = await fastify.inject({
+    method: 'GET',
+    path: '/'
+  })
+  t.ok(response.headers['cache-control'])
+  t.equal(response.headers['cache-control'], 'do not override')
+})
+
 test('only sets max-age and ignores s-maxage with private header', async (t) => {
   t.plan(2)