My answer is keep both. If you send one back, then the one you keep WILL end up having horrible problems and you'll have a whole hassle and the company will just end up replacing the one you kept, and probably with the one you sent back.
Getting two should be considered a gift from Providence/God, who knows best what the future holds. He thought you needed two. Who are YOU to question His infinite wisdom?
BUT my final answer? Return the extra MacBook. The fact that you are asking "What should I do" shows that you would feel guilty about keeping both. If you keep both you'll always feel guilty about the second and never be able to enjoy your fortuitous free MacBook. Just send it back. Or, you could try calling Apple and see what they say. Maybe they'll say, "Um... nope just checked our inventory, we didn't send you anything extra. So whatever you got it's yours." At this point you can argue with them that you most certainly DID receive something extra in your shipment, but if you do that then just send both MacBooks back because you clearly don't deserve either of them.
