Here's why:
* One-time use: Wonder Mail codes are designed to be used only once per game save file.
* Preventing duplication: This prevents players from getting the same items or rewards multiple times and keeps the game balanced.
Important Note: This applies to all Mystery Dungeon games, including Pokémon Mystery Dungeon: Red Rescue Team, Blue Rescue Team, Explorers of Time, Explorers of Darkness, Explorers of Sky, Gates to Infinity, Super Mystery Dungeon, Rescue Team DX, and Mystery Dungeon DX.
If you want to get the benefits of a Wonder Mail code again, you'll need to find a new code or start a new game save file.