No offense to anyone, but there's a lot of stupidity in this thread. I'm just going to make 3 points:
1. OP seriously needs check his account to see if 2 lines are set up. Since he was charged for both phones, this is extremely likely, whether due to AT&T error or the OP screwing up his order (see it all the time). If that's the case, he's definitely going to be caught, because if the second line is never activated and the device isn't returned, he'll be charged for the equipment. He doesn't need to call AT&T do do this, just go to att.com/mywireless
2. If he somehow managed to be shipped two phones with only a single line (very unlikely), he'll still probably be caught, because all shipment info is recorded on the account.
3. Even if (big if) the phone was shipped accidentally and even if (bigger if) OP is actually legally entitled to keep it, when AT&T catches the error they'll still charge him full price, and the charge will appear on his monthly bill. At that point OP has 4 options - return the phone (if he still has it), pay the charge, don't pay and let the account be canceled (with credit rating hits and so on), or sue AT&T to have the charge removed. Is OP willing to go through one of the latter two options to get a "free" phone?