Most likely under United States law you're not required to return it. It may be considered an "Unsolicited Item." You can look it up online if you're interested, but there is legal support for this ability.
If you "requested or solicited" the item, you may be required to notify the sender that you have it. They will need to make arrangements to have the item picked up and returned to them. You are not required to pay for its return.
Unless you placed two orders, I think it would fall under the former, but AT&T/Apple would argue the latter. Obviously the moral thing to do is to contact the seller for its return, but...
If you "requested or solicited" the item, you may be required to notify the sender that you have it. They will need to make arrangements to have the item picked up and returned to them. You are not required to pay for its return.
Unless you placed two orders, I think it would fall under the former, but AT&T/Apple would argue the latter. Obviously the moral thing to do is to contact the seller for its return, but...