I don't know how one officially complains to Apple.
http://www.apple.com/feedback/
Keep a note of device serial numbers though -- it'd be funny if they sent out the same iPad for your second and third orders!
Returned iPads are sent back to the factory, tested, given a new battery and I believe a new back, and stamped with a different serial number before being sent back out. So no, you'll never get back an iPad with the same serial number as the one you returned.