Quick answer: Each eShop code can be redeemed only once. If you see this but never redeemed it, stop retrying — first check whether it was already added to one of your accounts, then contact the seller with your order details.
An eShop download code works exactly once. This message means Nintendo's system sees the code as already redeemed — which is usually explainable, and fixable.
The most common explanation is that the funds were already added — maybe on a different user profile, or by the gift recipient. Before anything else, check your eShop balance on each of your accounts to see if the credit is already there.
If the balance genuinely isn't on any of your accounts, don't enter the code over and over. Repeated failed attempts won't change the result and can temporarily lock code entry on your account. Make sure you're also typing it exactly as shown — see card not working for the common typos.
If you bought the card from us and it shows as used but you never redeemed it, reach out with your order number. We'll look into it and make it right — that's what the support team is here for.