Now it's happening again ....
Today I caught the same user cheating, we were competing in a local contest held on CF after which I could realize that he's using my codes.
Actually this happened when we met once and I logged in using my account on his laptop, maybe he's using a key logger or something.
I reviewed some of his codes in some rated rounds and I can see no difference between his codes and mine.
I admit it's my mistake to fall on me such a classical trick, but it actually happened!