What does "This Code Has Already Been Used" mean on Nintendo?

This message means your 16-character Nintendo download code has already been redeemed on a Nintendo Account and cannot be used again. First check your Virtual Game Cards on the HOME Menu — the game may already be in your library from a previous redemption. Also check any other Nintendo Accounts on the same console. If the game is not on any account and you entered the code correctly, the code was redeemed by someone else. Contact your seller with proof of purchase and request a replacement or refund. Note: Nintendo cannot reverse a code redemption once it has been used.