optimization when adding child groups

lib/ansible/inventory/group.py

@@ -35,6 +35,9 @@ class Group(object):
 
         if self == group:
             raise Exception("can't add group to itself")
+
+        # don't add if it's already there
+        if not group in self.child_groups:
             self.child_groups.append(group)
             group.depth = max([self.depth+1, group.depth])
             group.parent_groups.append(self)