Ordinii, if a new account is rejected for whatever reason, it no longer exists, so that is not the issue.
Guest, did you try making the second attempt in a public place like an internet cafe, or school? If you did its possible that there are other players that logged in there and it was throwing up their names as being a multi. If you are doing it in such a place, there is a multi field so just put that you are in a internet cafe etc. That should help.
If you were doing it at home, or somewhere else where you don't expect random people, just ask around those that do have access and see if they play - they may have been playing without your knowledge. If this is the case, just put their character's handle in the multi field and it will be dealt with.
If there is still an issue, make a support ticket about it, and make a new account and in the multi field say to check the support ticket (Contact Support link at the bottom right).