To the OP: If you ordered your laptop directly from Apple.com, and they sent you two (one by mistake) maybe you should check your online transaction record at apple.com and make sure your credit card hasn't already been charged... twice. If somebody duplicated the transaction, it might just have been duplicated everywhere... Two 17" MBPs, two charges on credit card. Just a thought, but I'd definitely check to make sure if I was you.
It would be really a shame if somehow you returned the "extra" MBP, only to see a nice charge on your credit card bill the next month for the same machine... There are people who order more than one at a time, so this is one explanation as to why you got two shipped to you. Somebody typed 2 instead of 1 somewhere.