If they're doing their jobs correctly, they shouldn't take anything in that trades for more than it sells. For example if you brought in FIFA 07, 08 and 09 which may trade at 50p each and sell for 99p, they shouldn't give you the promotion price, as that would make the trade value of each game £2.83 and they'd make a loss.Yeah, absolutely no duplicates and the staff have the right to refuse if they feel you are trying to abuse the offer.
Also that promo copy won't (or at least shouldn't) be accepted.
That's if they're doing their job correctly.