The parent claimed that it was impossible to test for the existence of a copy and that therefore its existence or lack thereof is not relevant. I claimed that it is possible to test for the existence of the copy. I would further claim that there absolutely is a practical difference as far as a renter is concerned, whether or not they test for the existence: The person who copied the rented DVD is very unlikely to rent it again when the want to watch it later. Which I presume they intend to do given that they went to the (admittedly minimal) trouble of copying the movie in the first place. I suppose that in the scenario where it is a lent item, the argument of lost revenue is slightly less strong.