incrediBILL - 12:45 am on Oct 19, 2012 (gmt 0)
From the FTC's website:
Q. Am I obligated to return or pay for merchandise I never ordered?
A. No. If you receive merchandise that you didnít order, you have a legal right to keep it as a free gift.
Tell him you're going to charge for the second shipment unless the first is returned.
That could get you in boatload of trouble and be considered FRAUD.
The mistake was shipping the correct item before the mistake was returned.
However, I probably would've done the same thing.