Nope, I am pretty sure your analogy fails.  If you receive USD in the mail and have no idea who the sender is, then you obviously can't send it back.  But you know who the sender is, and have acknowledged as much yourself.  So you can send it back, and you ought to, both morally and legally.  Or else the scammer tag is well placed and you can live with it.

I have had several people accidentally pay for Casascius Coins twice.  I have always refunded the overpayment, no questions asked.