diff --git a/lib/ansible/modules/system/user.py b/lib/ansible/modules/system/user.py
index 49a71d99a71..fb0754183d7 100644
--- a/lib/ansible/modules/system/user.py
+++ b/lib/ansible/modules/system/user.py
@@ -552,8 +552,8 @@ class User(object):
         if self.groups is None:
             return None
         info = self.user_info()
-        groups = set([x.strip() for x in self.groups.split(',') if x])
-        for g in set(groups):
+        groups = set(x.strip() for x in self.groups.split(',') if x)
+        for g in groups.copy():
             if not self.group_exists(g):
                 self.module.fail_json(msg="Group %s does not exist" % (g))
             if info and remove_existing and self.group_info(g)[2] == info[3]: