Prepaid time card

I just purchased a 2 month prepaid game time card at Wal*Mart. I checked the back, as I always do, and it was unscratched. I brought it home and scratched it off to put the code in on the account management page, and to my surprise, it told me that the code had already been used. As I said, I checked the card before I bought it (I almost bought a scratched off one before, so now I always check), and it was untouched.

I've typed it in twice, and it turned out the same, telling me that the code had already been used. At this point, I'm clueless, and could REALLY use some GM help :(

I've already had to argue with Wal*Mart before over a time card (they didn't swipe it right, so it wouldn't work :/), and REALLY don't want to have to go through that again.