I posted this on SDA already, but I thought I'd put it here for posterity. This works for any battles including overworld encounters and boss battles. I believe it's either slightly faster or slightly slower than a first try Platoon, but there's very little to no RNG in doing the glitch so no more resetting 3+ hours in if the AI just completely shafts you at Platoon.

Items duplicate based on the order in which you received the items in your inventory.

To duplicate items:

1. Get into an encounter and use up all of any one item
2. Win the fight
3. Get into another encounter and die
4. Continue and you should have duplicated the last item you received (so if the last item you received was the Great Sage's Secret and you had 3 total at the start of the first encounter, you'll get an additional 3 after continuing)

There are also a few properties to the glitch that I've found after some testing:

1. If you completely use up X items and perform the glitch, you duplicate the last X items you got in your inventory (haven't tested what happens if you say get 3 coffees, buy a bunch of different items, then buy 1 more coffee)

2. If you use up 1 item in a battle and win, then use up X more items in another battle and win, the game saves the items that should get duplicated. ex: If the order you received items was White Bread, Poison-Be-Gone, Blindness-Be-Gone, Curse-Be-Gone, and Iced Coffee and you use up the White Bread in the first encounter and then the Poison and Blindness-Be-Gone in the next encounter and ¤then¤ proceed to die, you will get 2 iced coffees and 1 Curse-Be-Gone. (Haven't done extensive testing on using items up in weird orders, but I did do one weird combination in the video and I think I have a grasp of how the game processes the glitch)

3. If you get a provision after a battle, it can screw up the setup. (I haven't done extensive testing on this, but I did get the glitch to not work when I used White Bread for the glitch and got another White Bread at the end of the encounter)

With duplication, it means the run would require you to get into a bunch of encounters at Old Smoky to duplicate the Great Sage's Secrets 4-5 times. I think 5 times might be ideal as you can sell just 3 of the secrets first and buy the Coffees you need then sell a few more for Cappucinos, and then use the rest of the secrets for the rest of the game (which would probably somewhat trivialize the endgame bosses since you would fully heal and restore MP per use instead of just recovering one or the other) although I don't know what would be the most efficient way to route in duplication.

Just a quick update, after some more testing I found a few things:

1. You don't have to actually win the battle, you can just flee instead. The battles at Old Smoky aren't too hard though, so winning the fights may or may not be better for the extra exp and the safety of not failing escapes multiple times in a row.

2. The order of the inventory might actually make some difference. It only happened once for some reason, but when I bought the Great Sage's Secret at the Casino while I had an empty space, it duped the other items instead. I tried buying a Curse-Be-Gone at the store with an empty space but it still treated it as the newest item, so maybe the Casino is a special circumstance?

3. I said the endgame bosses may be somewhat trivialized. Scratch that, this game is still pretty hard. It's just slightly less hard. :P

Going to do some re-routing before I do another attempt, but a few things that definitely seem useful:

1. Buy Pixie Dews instead of Sandwiches at Yule. 100 healing to all party members is so worth the extra cost now that money isn't an issue.

2. Don't be afraid to use Great Sage's Secrets when you run out of items or are at the final few bosses. I used 2-3 at the Moltaan fight alone, and after buying a bunch of random items on the fly I still had 20+ Great Sage's Secrets left along with a ton of extra items. Could really cut out some of the cappuccinos and use Great Sage's Secrets at some point.

3. Bluster Blade at Red Dragon is too good. Might be worth the trip to the Casino just for it. Would cost about 8 Great Sage's Secrets to afford it. I think that alone makes first try Platoon faster at the moment (going to Casino and getting first try Platoon vs 96 Great Sage's Secrets is about the same time from what I've tested).

4. Need to look into buying other weapons. No point in having so much extra money if it's not even gonna be used.

TL;DR at the bottom

The glitch targets an issue the PS3 has with allocating memory based on 1's and 0's to unused inventory slots. Although the glitch isn't really centered around 'last item received' it's a better way of explaining it to people so for the sake of the guide we will use the phrase.

So we will say that the duplication glitch triggers on the last item received. However, the method for determining what your last item received was is not based on what you picked up last.

This is the method, based on memory bank storage in the inventory menu, that determines what will be duplicated.

Inventory in the game is read from left to right, and secondly by row.

For example we will do it like this (Item 1) (Item 2) (Item 3) (Item 4) (Item 5) (Item 6)

If, at any point, you USE UP any item... we will use item 2.... then it looks like this (Item 1) ( ) (Item 3) (Item 4) (Item 5) (Item 6)

It will not make item 3 item 2 and shift it to fill the space. This is crucial. If you receive an item through ANY means (purchasing, chests, spoils, etc) then it will then take the FIRST AVAILABLE INVENTORY SLOT. So, let's say you have an inventory that looks like this : (Item 1) ( ) (Item 3) (Item 4) (Item 5) (Item 6) and you get the sage's secret (Item 1) (Item 2/Sage's Secret) (Item 3) (Item 4) (Item 5) (Item 6)

The glitch will not work on the Secret. The game believes, still, that item 2 was received in the order it is displayed on inventory. The item being duplicated here would be item 6. Sage's secret becomes item 2, even though it was the last item received. In order to be the last item received, you need it to become item 7. (Last in the list, and a spot never - at any point - was held by another item)

So, in short, you need to have all space that has EVER been held by an item in the game filled with something... anything. This moves the last obtained item automatically to the furthest right inventory slot and makes it the "last item received"

You would think that manually moving your items to make them appear like this would work, but it DOES NOT. (Item 1) (Item 2) (Item 3) (Item 4) (Item 5) (Item 6) (Item 1) ( ) (<--Item 3) (Item 4) (Item 5) (Item 6) (Item 1) (Item 2) (Item 3) (Item 4) (Item 5) ( )

The game still has item 6 in it's memory and whatever is next received after manually moving everything down in order will not become the "last item received" Instead, it will become item 6 which you have already had at some point. Pay attention to this.

Also, it is important to note that selling items in a store IS THE ONLY WAY that actually resets the games memory for that item position in the inventory back to 0, thus making whatever fill it's place next to be figured in proper order.

So, if my inventory looks like this (Item 1) (Item 2) (Item 3) (Item 4) (Item 5) (Item 6) and I sell some stuff (Item 1) (Item 2) (Item 3) ( ) ( ) ( ) The items that take that place will be considered "last items received" in the order they get listed in inventory because the act of selling an item is the one way in the game for the inventory slot to be reset to a "0" in the PS3 memory allocation bank. This is where the whole belief that it's always your last received item that is duplicated.

TL;DR This most important take aways from my poor explanation of this here is that if you used up any item before you get to the Sage's Secret spot, check your inventory. If there are ANY gaps in your inventory you need to go to a store or pick up an item to ensure that the spot is filled before Sage's Secret is received. I would recommend you keep at least 1 of every consumable item you receive until you get to your duplication point for extra consistency safety. Doing this eliminates any issue entirely that the last item you acquire will not, in fact, be the "last item received" Sage's Secret (or whatever you're duplicating) absolutely has to be listed last in the inventory menu and it has to be a spot that has, at any point in the game, NEVER been something else. Doing this will ensure that you get the glitch to work 100% of the time, every single time. However, if you have items and you sell some, the sold spots will then become 0's and it will no longer have mattered if there was an item at any point in that slot. As long as it is the last item in the list of inventory items and the previous item in that spot was SOLD, it will be considered the 'last item received'

Thanks to Qazplm9000 for his discovery, I got a tiny bit obsessed with breaking it down for a bit there.