When using Fabric Carpet mod to stack empty shulker boxes, and then taking that stack of shulker boxes and right clicking on an item in your inventory, it causes the item to be duplicated.

Steps to reproduce:

Have "stackableShulkerBoxes" enabled in Fabric Carpet
Have "supportsBundlingInsert" enabled in Quickshulker

Open up your inventory menu.
Stack however many shulker boxes, then pick those up with your cursor. Right click them on an item to pull it into the boxes.
Set down the boxes in their own inventory slot. Each box now has it's own version of that item.