What about purposely returning without the receipt when you have it? Or is that what you meant? If you really don't have the receipt then its totally Ta'os Akum. You can tell them about it but you don't really have to. Would it still be considered Ta'os Akum if you do it purposely? They are still making a mistake, even though you led them to do it. Sorry for rambling.
A little off topic but there are so many cases similar to this. What if you order something online and it doesn't come. Call them up and they will either reship it or credit you. Then you get the original package. Do you need to notify them?
i see what your question is regarding the reciepts and that i dont have an answer for - and its a valid question since you are not required to return with the receipt.
regarding notifying them or returning a double, i dont either know, but it has happened to me before, with a warranty company. i claimed the warranty on my ipod 80GB a few yrs ago since the battary was messed up and i sent it in 3 times back and forth and they couldnt fix so i told them it was around $200 - and they said theyll send me a check of $200.
later i got the receipt from sams club database and it was $230 so i PUTPAC again and they said theyll send a second check of $30.
in the end i got 3 checks! $200, $30, and $200. (and returned the ipod to the warranty place)