Multiple users with same name

[SIRprise]

A guest can enter the room with the same name as registered users/the admin. I guess there is somewhere a comparison against the extendended username (with pre/postfix).

[Frug]

Guest usernames are always surrounded in brackets () and in the provided themes are generally in a different color than actual users. We could add the option to disable sharing a name with a user, but for now I think it's fairly clear in most cases who's a guest and who's the actual user.