diff --git a/src/Fetch/Server.php b/src/Fetch/Server.php
index 32e57c1..c6d0d59 100644
--- a/src/Fetch/Server.php
+++ b/src/Fetch/Server.php
@@ -210,7 +210,7 @@ public function setFlag($flag, $value = null)
             if ($value == false && $index !== false) {
                 unset($this->flags[$index]);
             } elseif ($value != false) {
-                $match = preg_grep('/' . $flag . '/', $this->flags);
+                $match = preg_grep('/\A' . $flag . '(?==)/', $this->flags);
                 if (reset($match)) {
                     $this->flags[key($match)] = $flag . '=' . $value;
                 } else {