I tried to search for a solution for my problem, but couldn't find anything about my specific problem.
I installed Joomla just a couple of weeks ago, so I'm using Joomla 3.1.5 and every plugin is up to date. I use a lot of plugins, e. g. Kunena, CB and JEvents and more. When the users register, they have to click the activation link and then an administrator gives them access to the Kunena board.
Everything is working now, but my problem is that when a new user is registering and clicks the link, the admins get 2 emails with exactly the same content à la "someone registered, please give them access to the board". I hope this is a CB related problem, but I think so, because when I used the Joomla registration plugin, it worked.