I don't know about refusing to pay the items, but we can become numb to the fact that there are legitimate duplicates out there and miss a forgery because of it. I am a Yellow Hammer user and I've called customers on that and explained that I simply can't catch fraud using that method if they continue to use the duplicates. Refusing to pay the items based on the serial number sounds a little risky, though, because there are cases where the serial numbers aren't actually duplicates but an encoding error occurs and they are reported as such.