optimization when adding child groups

lib/ansible/inventory/group.py

@@ -35,9 +35,12 @@ class Group(object):
 
         if self == group:
             raise Exception("can't add group to itself")
-        self.child_groups.append(group)
-        group.depth = max([self.depth+1, group.depth])
-        group.parent_groups.append(self)
+
+        # 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)
 
     def add_host(self, host):