Not sure how this carries over into retail and e-tail business aspects.
But in the banking industry, payroll, and other financial institutions, it is Illegal for you to accept money or product in error and not report the error to the issuant.
for example. If you went to your bank account and found an extra $200 deposit that you know isn't yours, you have the legal responsibility to report it back to the financial institution, and it is at their descretion whether or not you get to keep it or not.
Should you fail to report, they can withdraw the amount from your account, and should it be unrecoverable, you can be sued and charged for theft.
This goes for payroll deposits and any financial transactions in north america.
Does this same legal precedence apply to receiving 2 items isntead of 1 on order? I'm not sure, but knowing how stringent and corporate friendly laws tend to be, I am willing to bet the same.
A lot of people get small items accidentally and chances are for Amazon, it's not worth going after them to return a $20 t-shirt. But in the realm of a $1,400 laptop, you may find yourself receiving notice to return it sooner than later if they catch their mistake. I think the best thing you can do is call them directly and let them inform you of what they want to do. If they say 'keep it', consider yourself lucky. But until you call them, you are in affect being morally suspect