diff --git a/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php b/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php
index b36b7121e..80498b737 100644
--- a/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php
+++ b/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php
@@ -2044,7 +2044,7 @@ class ClassMetadataInfo implements ClassMetadata
             if ( ! class_exists($className)) {
                 throw MappingException::invalidClassInDiscriminatorMap($className, $this->name);
             }
-            if (is_subclass_of($className, $this->name)) {
+            if (is_subclass_of($className, $this->name) && ! in_array($className, $this->subClasses)) {
                 $this->subClasses[] = $className;
             }
         }