Only add a User to the Public Group if they aren't already a member

refinery/core/models.py

@@ -133,16 +133,15 @@ def create_user_profile(sender, instance, created, **kwargs):
 
 
 @receiver(post_save, sender=User)
-def add_user_to_public_group(sender, instance, created, **kwargs):
-    """Add users to Public group automatically
-
-    """
+def add_user_to_public_group(sender, **kwargs):
+    """Add users to Public group automatically"""
+    user = kwargs["instance"]
     public_group = ExtendedGroup.objects.public_group()
     # need to check if Public group exists to avoid errors when creating
     # user accounts (like superuser and AnonymousUser) before the group
     # is created by init_refinery command
-    if public_group:
-        instance.groups.add(public_group)
+    if public_group and user not in public_group.user_set.all():
+        user.groups.add(public_group)
 
 
 def create_user_profile_registered(sender, user, request, **kwargs):