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SCIf someone wasn't greedy, all you'd need to do is aquire (create or get them backdoor from the current supplier) unused plastic holders. Then, buy decent looking examples of cards that are at the grade you want and grade below, crack them out (without damaging the flip) and switch them out. Submit the now raw card and you'll end up with 2 PSA 8s out of a PSA 7 and PSA 8. Buy another PSA 7 and repeat. Buying a sonic welding machine is nothing - I think Lelands had one in a past auction?
I'm pretty sure this is what WIWAG did, or similar. Without recreating flips, you have no worries about duplicate/invalid numbers. No worries about the flips not looking right. As long as you're picking nice cards, you're not even going to have a huge problem with the cards themselves, as they're real and not terribly overgraded.
Think about it - there have been piles of books written on artwork forgeries, with top experts disagreeing on the veracity of paintings worth millions. What defense is there for things as simple as cards and coins?