It sounds like Nintendo should ask the courier company to fix things, not you. In a way, he is also a victim of the courier company's bad service, except that the mistake was in his favor.
And also, think about what that implies. How far are you supposed to go if they screw up? What if they do it again, with something more valuable?
You can't apply human morality to large corporations. If someone on Etsy or Ebay made a mistake like that, then yes of course you should ship it back. But corporations of that size are just systems. They will not be hurt or offended if you don't return it, and they won't be gracious if you do.
I was surprised by this - I thought it would constitute unsolicited goods but it does not due to the extra item having been sent as remedy for a broken item. If someone sends you something out of the blue and then demands payment it is unsolicited goods and you have the right to keep it.
But as the above comment says, you would have had to send the item as part of an existing engagement, such as a purchase or replacement.
As long as you don't use them the sender has no right to redress, only to ask for them back.
Ok. What if my payment processor has an issue, through no fault of the seller, and I end up getting charged twice? Do your rules still apply? It seems unreasonable to just assume a 'screw you' attitude from the get go.