If the code has been checked more than once, then you can only be sure that >=(n - 1) counterfeits exist. Checking the code does not verify whether yours is the counterfeit, only that you weren't the first to attempt verification.

If the code is behind a scratch off strip, then because you scratched it off and the code was already used, you can be assured that your item is counterfeit.

On the other hand, being the first to check the code is NOT perfect proof that your item is legitimate. If the counterfeiter makes 100 of an item, the one of those will still pass verification. Unless you get notified of other attempts later, of course.