Yes, if you cash the second check knowing (as you evidently do) that you are not entitled to it, you will be committing theft and could be criminally prosecuted: it doesn't matter if the other side made an error in sending you the money again, the fact that *you* know that you were already paid means that you know you have no entitlement to the funds--i.e. that you will have committed theft.You could also be sued for the money's return.