"Username taken" system

People have registered for my forum but a lot always had the same username so I had to e-mail them to change their username. It would be nice to have a system that will not accept the same name more than once.

The system as it is, does not accept the same username. I don't understand what are you saying.

Yes. The system now will not allow the same username. Do you mean that two members could not have usernames like this: slg and slg73?

Before, two members signed up, both with the name Iconlite123. I had to tell one of them to change their username to something else and he did. Since the both had the same username i don't think this system was working then.