diff --git a/lib/Doctrine/ORM/Mapping/Driver/DriverChain.php b/lib/Doctrine/ORM/Mapping/Driver/DriverChain.php
index 77c258a18..d84d2344b 100644
--- a/lib/Doctrine/ORM/Mapping/Driver/DriverChain.php
+++ b/lib/Doctrine/ORM/Mapping/Driver/DriverChain.php
@@ -88,15 +88,20 @@ class DriverChain implements Driver
     public function getAllClassNames()
     {
         $classNames = array();
+        $driverClasses = array();
         foreach ($this->_drivers AS $namespace => $driver) {
-            $driverClasses = $driver->getAllClassNames();
-            foreach ($driverClasses AS $className) {
+            $oid = spl_object_hash($driver);
+            if (!isset($driverClasses[$oid])) {
+                $driverClasses[$oid] = $driver->getAllClassNames();
+            }
+            
+            foreach ($driverClasses[$oid] AS $className) {
                 if (strpos($className, $namespace) === 0) {
-                    $classNames[] = $className;
+                    $classNames[$className] = true;
                 }
             }
         }
-        return array_unique($classNames);
+        return array_keys($classNames);
     }
 
     /**