I don't know where should I report this situation so I am writing this blog.
ppavic, this guy has fear of losing his rating on some rounds, so he is using his second account, fbosnjak_, for those rounds. I know that because I was with him today in room during the contest and some other codeforces members (jklepec, Gabrijel, ...), which were with us in room today, can confirm that.
I hope that admins will ban his accounts.
P.S. Real Fabijan Bosnjak's account is fbosnjak.