Serial Help

[Batrakio]

I created a server and my cousin was going to join, when he tried to join he gets that "Bla,bla,bla,...CD-Key in use,Bla,bla,hijack" thing!
No one was in the server except me and we do not share the same serial!
What can he do?

[DOGGiE52]

Then someone could have taken his serial key. Otherwise I really don't know.