I agree with mikedt, it's probably not fake. However, you can check yourself. There is an app in the Play Store called Genuine Galaxy S4/S3/Note II that is free and well rated that will allow you to check whether your phone is genuine or not.
I would be very surprised if it is not genuine, but less surprised if someone has tampered with the IMEI after it was stolen.
Out of interest, which IMEI is the valid one?