User has already been registered

I get this message for two new users I try to register. Why? How come?
I doubt they are Clicky-users but I do not know. But even if they are existing users (for some other site) I'd like to add them to our sites.

Sub-user accounts share the same database as normal users, so that means that someone else who created an account on our site already took that username.

Thanks,
Suggest that you change the message to say the the "user name" is taken and not the more generic message used today.
Cheers

I checked and it does say that. I did edit it slightly to make it more clear though.