Interesting! I didn't know this. Unfortunately in this case the buyer is in the UK, so FTC rules don't apply to him. I hope they have a similar consumer protection rule.
You don't have to send unsolicited goods back or even notify the sender.
But duplicate orders don't count as "unsolicited" and should be returned - however "there should be no cost or inconvenience to you" which is where Nintendo fail badly here.
