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I would have simply refused delivery when it arrived at my doorstep, and then disputed the credit card charge if Apple attempted to bill me for it in the future.

Since you accepted delivery (or somebody else in your family did), then I'd keep it. Selling it for anything close to face value will end up ripping the buyer off in the end, unless you tell them up front that Apple has no record of the Serial number, and as such they will not be able to get an RMA for repairs in the event they're needed, nor will they likely be able to purchase AppleCare for it.

You could also try bringing it by your local Apple store or something ... maybe they can give you a store credit for it, or let you exchange it for something else (worth a shot, at any rate) ... provided you didn't open it yet.
 
I would email executive relations, telling them what you have attempted to do so far and what the CS rep told you; that way either they will get it straightened out (higher up the food chain than the CS rep), or they will tell you to keep it, but if you have it in writing you will have that later should lines get crossed and Apple comes knocking later. Right now all you have is word of mouth over the phone that it can't be returned, but if you keep it/sell it and Apple comes looking for it later, you won't have much of a leg to stand on if you don't have anything in writing saying it's ok to keep it, or at least that you attempted to return it.
 
Donate it to charity. Raffle it on Ebay and give the proceeds to charity. ....
 
Why not send the one back that is on record. Keep the 2nd one, in a few weeks I am sure you can figure it out with apple how to pay for it.:p
 
I had a similar thing happen to me when I ordered my iPod Nano 5th gen. It didn't arrive within the time frame, so I phoned Apple and they sent another. That one arrived and then suddenly the original arrived a few week later. I also phoned Apple, out of fear of being legally shafted otherwise, they told me to just keep hold of it, so I ended up selling it for a tidy profit. My advice is to embrace the good fortune since you've covered your bases.
 
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