Some will point out (and some have!) that I could have quite easily saved myself a lot of time, hassle, and lost earnings at this point by just quietly holding on to the extra Switch and saying nothing to Nintendo about it. It certainly would have saved me a lot of stress, but it was morally not something I’d ever consider.
That's his mistake right there. He was under no moral obligation to fix Nintendo's mistake for them. Nintendo had made a string of mistakes up to this point, and one of their mistakes was in his favor.
If you truly have a guilty conscience, then just throw the extra package out, because it is abandoned property.
I've had this happen occasionally. Just know that you are not responsible for stuff like this. My Dad had a similar experience, where spent weeks trying to get a company to honor a warranty, and in the end they sent him two replacement hard drives instead of one. I actually interpret this as a kind of karma. Sometimes good things land in your lap, don't reject them.