I have myself at Access Level 1 and 4 others with level 2 access and all work fine. A total of 5 users.
They are active only one a time.
However when I try to add a 6th user, I get a message "User name must be unique" and then "not saved".
Strange because the User Name IS unique (it's a call sign) and I used pHpMyAdmin to look at the users list and there are only the 5 users shown.