This is done by equipping the item to be duplicated (must only have one of that item) to Labyrinthia and then entering a Free Battle against a Harpy. Have Labyrinthia create a Replica in front of the party and wait for a Harpy to use 'Steal Item' on the Replica. Then have the original Labyrinthia use her copy of the item and complete the battle to get '0%55' (255) of that item which can then be sold for easy money. Some items in your inventory may become inaccessible in some menus but this can be fixed by saving the game and reloading it after you are done selling.
When you have completed the game you will be able to start a new session with the cleared game file and earn double the Experience points, Gella, and CSP. Each new session with the cleared game file will increase this bonus by one. For example on your third session the value will be triple.