I do wonder if they’ll check if you have played a game via the MIG offline then go online later with a legit game, I don’t think they do currently but with the existence of the MIG I am certain Nintendo will make that happen with an update.
The MIG uses its own IDs from what I understand, and if they’re invalid (randomly generated) or used by others (not random and used by other MIG users) Nintendo will know, autodetect, and ban you
That’s a third party multi cartridge device, not related to the MIG cart at all tho.
Instantly if you try playing online.
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