To duplicate items, equip the item to be duplicated. You must only have one of that item. Get into a Free Battle with a Harpy, which can be found in Bandit's Forest. Have Labyrinthia create a Replica in front of the party, and wait for a Harpy to use "Steal Item" on the Replica. Have the real Labyrinthia use her copy of the item, then finish the battle. You will now have 255 (appears as 0%55) of the item to sell. The Gella Card is worth 380,000 each time. Repeat this process as many times as desired. Note: Other items in your inventory may become inaccessible in some menus. To fix this, save the game and reload it after you are done selling.
Complete the game and load your cleared saved game file to start a new game and earn double the experience points, Gella, and CSP. Each new session with the cleared game file will increase this bonus by one (triple the value on your third session, etc.).