It's their mistake, don't tell them. Give it to a friend, or flog it on eBay. The bastards deserve it after the way they messed up release day shipping.
I know it's wrong to keep it, but a good friend of mine wasn't able to buy Skyrim because his mother confiscated all of his money because she thought he was smoking marijuana. He
was not smoking anything. He's a good person, and it's not his fault his mom is paranoid and only trusts his sisters.
I'll probably give the spare to him once I confirmed there's only a charge for one copy on my credit card.