Card or code is already redeemed
Card or code is already redeemed error. Signed out of all devices and signed back in. Still no luck.
According to: If you can't redeem your App Store & iTunes Gift Card, Apple Music Gift Card, or content code - Apple Support
"Card or code is already redeemed
If you see a message that says your card or code has already been redeemed, try signing out of the iTunes Store on all of your devices, then sign back in. This will refresh your credit balance. Learn how to find your credit balance.
If you redeemed your card toward an Apple Music membership, you can view your Apple Music membership and when you'll next be charged.
If you need more help, contact us."