Just check your download history
Thousands of people have recently picked up a Nintendo 2DS or 3DS (good choice, by the way!), and as a result, there have been a ton of issues with every facet of the eShop. One issue in particular involves the constant error codes that have plagued folks who have attempted to redeem codes.
Thankfully, Nintendo has offered up a possible solution. If you encounter the errors "Error Message: This Code Has Already Been Used," or "A Download Code is not working because it has already been redeemed," check your download history first (eShop > Menu > Settings/Other > History > Your Downloads).
It's a simple solution, yes, but one that many may not think of as they're frantically trying to redeem their brand new codes.