Wild ARMs: Crossfire
This game is titled Wild ARMs XF in North America and Wild ARMs: Crossfire in Japan.
Successfully complete the game. You can start a new session with a 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 (triple the value on your third session, etc.).
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Equip the item to be duplicated to Labyrinthia. You must only have one of that item. Enter a Free Battle against a Harpy. Have Labyrinthia create a Replica in front of the party. Wait for a Harpy to use "Steal Item" on the Replica. Have the original Labyrinthia use her copy of the item then complete the battle. You will now have "0%55" (actually 255) of that item. You can now sell that item for easy money. 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.
Strategy guides from GameFAQs