diff --git a/app/models/group.rb b/app/models/group.rb
--- a/app/models/group.rb
+++ b/app/models/group.rb
@@ -10,7 +10,7 @@
 
   def add_users_skip_existing(users_list)
     new_list = []
-    users_list.each do |u|
+    users_list.uniq.each do |u|
       new_list << u unless users.include? u
     end
     users << new_list