In Pirohiko's TAS, an item duplication glitch is performed to obtain a very large stack of Grimoires. This glitch can be performed RTA, and allows us to duplicate all types of equipment, not just weapons, as well as consumables.
To set up the glitch, you must first have two separate stacks of the item you wish to duplicate. Then you must enter the item exchange screen, the one that appears whenever you obtain an item from a trapped chest, an item drop from a battle, or if you have a full inventory and obtain a new item.
On the exchange screen, take one of the item stacks and place it in the treasure section (the upper portion). Take that same stack and place it onto the second stack in your inventory. Then take the blank spot where your stack was in the treasure section and place it in your inventory.
This blank spot is actually a stack of 0 of your item. If it is a piece of equipment, you can equip it and then you will be left with a stack of 255. If it is a consumable*, in the next battle you enter, open your inventory and you will see 0 of the item. Use it and you will then have 255.
I believe that throwing stars could be duped by the same technique as consumables (darting them in battle to use one), but I have not tested this.
*The consumable part seems to only work in the 1.0 English release or the Japanese release. In 1.1 (and thus also Free Enterprise), the item will not show up in battle. However, you can still sell the stack of 0 for a large amount of gold (like with the commonly used weapon dupe, entering battle will cause you to lose the stack).
Q: Help, I want to dupe my Elixirs. How can I keep them from combining into a single stack?
A: When you obtain an item from a chest, it will by default add to any existing stack. To circumvent this, you can visit the Big Chocobo, give him your first Elixir, then go obtain the second Elixir. When you take the Elixir back from the Big Chocobo, it will be a separate stack.
Video demonstration: https://youtu.be/utIVOJekE8c