Quoting pinger80 who reported this on my pack. I've managed to replicate it.

i've found a pretty major dupe bug.
It's done on the vanilla crafting table, and allows you to replicate any craftable item, craftable as in can be made inside a vanilla crafting table (ex. resource blocks or craftable supplies like golden apples), but isn't limited to vanilla-only content.

Replication:
You do the bug by crafting the item as normal, but filling each inventory slot so that shift-clicking the item doesn't return it to your inventory.

After shift-clicking, you can freely craft a stack of the item if you keep clicking on the crafting result. Once you have a stack, and drop it by throwing it outside the window, clicking on the crafting result again rapidly increases the crafting result up to 64, or whatever the item's stack limit is, or whatever the original crafting result's closest multiple is to 64 (only goes up to 48 for sugarcane -> paper).

The progression of which the items multiply seems to be exponential, with regular stackable items going via a progression of 1>1>2>4>8>16>32>64, while other items such as the paper recipe go 3>3>6>12>24>48. The bug doesn't work if all the items in the crafting grid aren't consumed. (ex. if there are buckets of fluid which are not consumed by the recipe, like an infinite water source block)