Already redeemed gift card

i BOUGHT AN ITUNES GIFT CARD AND THE PERSON I GAVE THE NUMBER OFF THE BACK OF THE CARD WENT TO TRY AND REDEEM THIS CARD TOLD ME THAT THEY CANNOT USE IT BECAUSE IT HAS ALREADY BEEN REDEEMED. WHAT CAN I DO OR IT IS NOT POSSIBLE TO DO ANYTHING AT THIS POINT.

[Re-Titled by Moderator]