iTunes gift card already redeemed

Please follow the steps in this Apple Support article. 

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

If after reviewing the article, and following the directions in the article, you need additional assistance please follow the link below to contact official Apple Support. 

Contact Apple Gift Card Support

If the retailer has the Gift Cards displayed out in the open, then you were most likely the victim of a common scam that affects those retailers. Gift Cards should only be purchased behind a counter where they are not accessible to the public.

The scam works by anyone being able to grab a bunch of cards off the rack and then write down the code after scratching off the sticker. They scratch off stickers can be easily purchased at places like Amazon and they will then apply another sticker to the card. Then they will return the cards to the display rack and wait for someone else to activate at the checkstand. After that happens, they will spend the amount before you get a chance to.

The retailers know that this is a common occurrence, but some do not seem to care if their customers are scammed out of their money, even though they will lock up other products they sell to prevent theft where they will lose money. In addition to the link viewed on the top of this page, I would go to the retailer to complain. Apple may be able to help by using that link, but in the future never buy Gift Cards that are displayed so the public can tamper with them.