#25 deduplicate header links (#26)

Author: Nuranto

src/Middleware/AddHttp2ServerPush.php

@@ -50,6 +50,7 @@ protected function generateAndAttachLinkHeaders(Response $response, $limit = nul
                 return $this->buildLinkHeaderString($url);
             })
             ->filter()
+            ->unique()
             ->take($limit)
             ->implode(',');