The only time we would cancel someone's account for cheating for something like that is if we found out that they were doing it intentionally. (we have some various tracking tools). If you signed up and they told you that they were already signed up and we just didn't give you credit for it because WE didn't get credit for it, you would be fine. The big problem is when people sign up for things through another rewards program and get credit and then send us their confirmation emails and try to get credit from us as well, pretending that they signed up through us. 99% of the time, the merchant can tell us that you already got credit from another rewards program and are just trying to scam us.
As long as you are honest, all is cool.