If the stack size is over the 64 in the case of cobblestone, if you put it direktly into the offhand the items are getting duped

That's weird, I tried reproducing it in my dev environment and it's working correctly there.
What versions are you on?

Modpack

1.20.1-3.20.11.26 backpack
1.20.1-0.6.33.31 core
Fabric 0.16.7

No clue what mod could be causing this, if you find out let me know and I can look into what is causing it.

I still have no clue why it happened but after updating all mods the dupe does not work anymore