If you go to the account creation form (http://localhost/index.php/Special:UserLogin?type=signup&mobileaction=toggle_view_mobile) and you're already logged in as someone, you get the following info message:
"You are already logged in as Florian. Use the form below to log in as another user."
It's not correct, that the user can login there, so the message should be something like:
"You are already logged in as Florian. Use the form below to create another user."