failed to create a username

hello!
I invited somebody to my forum , he go there and try to register but he get an error like username is used before??why?
please tell me how to solve this problem
my forum allowed new members.

Spam-Link to Warez site removed.

This only appears when there is another user on your forum with that username. A common mistake with new users is that they click the register button twice fast, the first click adds the username, and the second click then finds that the first click registered the username so it says there is already a user with that username.

Its a bit confusing... but I hope this helps