My gift card has already been redeemed but I just bought it

A "Card has already been redeemed" messages is mentioned in this support article If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

"If you see a message that says that your card has already been redeemed, you might have already redeemed it. Sign out of the App Store or iTunes Store on all of your devices, then sign back in. This refreshes your Apple Account balance. Learn how to check your Apple Account balance.

If you need more help, contact Apple Support."

Try contacting them on a business day.

Note: I presume the number provided in an earlier post is the USA number. If you are not in the USA then have a look at this list of telephone numbers: Contact Apple for support and service - Apple Support

See the section about that issue in the Related Article above.

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Contact Apple Support as described.

Limnos: J_76 asked from that article.

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What did Apple say when you contacted them?

1-800-MY-APPLE