This glitch nets you an unlimited amount of Checks (AKA Charms) of any kind. The trick is to sell them so you can max out your Gald and Stamp Cards quickly (which is good for early Rare weapons and armor). First, select e a Check to duplicate (e.g. Curse Check). Have a total of 3, and equip them all. Then, select a Check to sacrifice (e.g. Poison Check). Have a total of 2. Now enter your Dualize menu and, for item #1, choose an equipped Curse Check. For item #2, choose the 2 Poison Checks and dualize them all at once. Now exit to the shop menu and go to Sell. You will have 255 Curse Checks and can sell up to 127 of them at a time. Your supply will instantly revert back to 255.
The glitched stock of 255 will remain in effect if you leave the shop, but there are 2 ways to correct it. The first way, in the shop menu, sell over 127 of them. (Note: Negative values expend Gald). You can also remove an equipped Curse Check. Note 1: You can have another character equip a Curse Check and reduce the supply to 254 without disabling the glitch. Also, this glitch can be done with other quantities than listed here, but 3:2 is the minimum. 4:3 is also possible; you just need to have one less Check than the type you want to duplicate.)