diff --git a/Foundation/CPSet.j b/Foundation/CPSet.j
index 16d9820ff..a44fbe2ac 100644
--- a/Foundation/CPSet.j
+++ b/Foundation/CPSet.j
@@ -372,6 +372,9 @@
 */
 - (void)addObject:(id)anObject
 {
+    if ([self containsObject:anObject])
+        return;
+
     _contents[[anObject UID]] = anObject;
     _count++;
 }