Can I use a Steam key if I already own the game?

Yes — if Steam tells you that you already own the game, the key is not consumed and can be activated on any other Steam account that doesn't own it. Simply log into a different account and activate it through Games → Activate a Product on Steam. If you bought the key by mistake and want a refund, contact the seller — since the key was never used, most legitimate sellers will process a refund or exchange.