I have started working with Expression Engine since last one year. Today i came across a problem in register page of frontend where my client wants to allow duplicate screen_name while any user register using register form(User module of Expression Engine). This is because user can login with username so there is no need for unique screen_name. Also when we use the register form of Expression Engine, we have to use screen_name and username(required fields). If you use username as unique entry than you can allow users to choose screen_name as per their choices.