How do I check if a PSN code has already been redeemed on my account?

On PS5 or PS4: go to Settings → Account Management → Account Information → Transaction History. On the web: sign into account.sonyentertainmentnetwork.com and check Transaction History. Wallet top-ups appear as "Wallet Funding" and game/subscription codes appear as the product name. If the code appears there, it was successfully redeemed. If it's not there, the code either failed or was used on a different account.