Usually when a code is getting redeemed it will get "locked" for a short period of time even if the given redemption is unsucessful in the end unless the redeem is speficially cancelled.
This "lock" should cancel after a while (15 or 30 minutes? i sadly dont know the current lock time but that's about it ) and if you are the only person trying to redeem that given code so you would be able to redeem the code again. To be safe maybe try a new browser session
SylvanFae: 1. check you library
2. ask for refund on site you purchased it
3. chargeback
4. don't purchase games on shady sites.
for now it sounds like the key is "locked" as in unsucessful redemption attempt and not "redeemed" / used for good by other person though"