not rocket science
All they need to do is let the users prove that they own a copy of the game on UMD by registering it against there PSN ID on a PSP 1000/2000/3000. Once thats been done you are given a redeem code to use on either the PS Store or PC Store to download a digital version.
Once the game has been registerd to a users account, it cant be downloaded again unless its that user re-downloading the game.