I had a few faulty discs in a large DVD box set, so i contacted the retailer and arranged a replacement.
The replacement set didn't arrive, so I waited a couple of days and they sent out a second replacement set.
The second replacement set arrived, then about 3 days later the first replacement (that had been declared lost) arrived.
Cue me ringing up and getting a CS rep who was, I think rather surprised that I was ringing up to return the "lost" set rather than just keeping it (she was ready to issue a refund for the set).
My general policy is, that if it's a mistake I'll try and sort it, if it's a mistake and they keep messing up I may reconsider but i'll always try to get it sorted, especially if the retailer has generally been ok.