DupeTrigger

This mod adds a keybind that doubles the item in your main-hand. The default keybind is "g", but you can change it in the Controls menu under the DupeTrigger section. It works when cheats are disabled.

There was a mod named DupeTrigger on curseforge, but it hasn't been updated to 1.21 due to the changes made in 1.21. I really liked this mod so I figured I would just make it myself. (This is my first mod)

The Stackable mod allows you to have more than 99 items in your hand when the game saves (it would crash otherwise). The item count will be saved upon closing the world.

Stackable also doubles the max stack size by default for every item. To fix this, go into

%appdata%./minecraft/config

and look for Stackable.json. Inside you can fix all the item stack sizes.