Steps to reproduce:
1 - access admin2 login page
2 - click on "Register new account" link
3 - fill the fields with:
       first name: First
       last name: User
       username: test
       password: test
       email: second_email@ez.no
4 - click on "Ok" button
5 - verify message "User registered"
(no need to verify on email)
6 - click on "Ok" button
7 - click on "Register new account" link
8 - fill the fields:
       first name: Second
       last name: User
       username: test
       password: test2
       email: first_email@ez.no
9 - click on "Ok" button
10- verify that it wasn't registered
11- verify message "User account: The username already exists, please choose another one."
12- verify that username is disabled