diff --git a/src/phage/agent/PhagePHPAgent.php b/src/phage/agent/PhagePHPAgent.php
--- a/src/phage/agent/PhagePHPAgent.php
+++ b/src/phage/agent/PhagePHPAgent.php
@@ -87,6 +87,7 @@
         'stdout'  => $result[1],
         'stderr'  => $result[2],
       ));
+    unset($this->exec[$key]);
   }
 
   public function __destruct() {