Me and my friend found a glitch to duplicate items into stacks.

Steps to reproduce the behavior:

1. Open backpack
2. Place a stackable item such as redstone into the blackpack
3. Place one of the same item into your inventory
4. Shift click the item from the backpack into your inventory
5. The item duplicates into a stack

Screenshots

My Mods

Version
1.16.4-2.2.6.205

Yes that version had this issue. I fixed it in a version released two days ago. Just download latest version and you are set. Btw your version of the mod is hidden on cf exactly for this reason.