My Apple gift card was already used by another customer

This link will take you to the section of a support article about what to do if your card has already been redeemed https://support.apple.com/108285#redeemed

If you purchased this card from a physical store it is possible that you are victim of a scam where a thief was ready to redeem the card once it was activated by the store. Avoid stores that do not keep their gift cards in secure locations or try to buy them online from Apple --> https://www.apple.com/shop/buy-giftcard/giftcard

More information about gift card scams at the very bottom of https://www.apple.com/shop/buy-giftcard/giftcard#:~:text=Beware%20of%20gift%20card%20scams

Ironically as the scam detailed where your money was stolen right in the store, some of those same retailers lock up products that can be stolen from them, but don't have the same concern for their customers. I have seen where customers were able to seek restitution from the retailer after escalating to Store Manager, District Manager, and finally the Corporate Office.

Only purchase Gift Cards from retailers where they secure their Gift Cards or purchase them directly from Apple.

Can we assume you bought it at a retail store off an aisle rack? Apple Gift Card scams are numerous. Thieves shoplift the card from the rack, carefully open it up and copy the redemption code, seal it back up, and return it to the rack. You buy it and the moment it is activated the thieves redeem it and you are out the money.