0. ALWAYS PROVIDE INFORMATION ABOUT MODLOADER (Fabric / Forge) AND MOD VERSION (For Example 1.19.2-8.2.10)

Fabric
1.19.2 - 0.4.18

1. Describe the bug

Items get duped when swapping backpack equipped with the exact same type of backpack (e.g. bat backpack for bat backpack)
The one youre putting on gets the items that are currently in your backpack duped into it

2. Write steps to reproduce the bug

Wear a backpack of any variant
Go into your inventory and pick up a backpack of the same variant, and swap it with the one you have on
Put the other backpack that's in your inventory on the ground
See that it has the same items

3. What is expected behaviour

Items will be duped

Mod version 8.2.17

8.2.19 is not available on Fabric 1.19.2

try updating to 8.2.19 firsst

Could you post a video of this dupe? I can't seem to replicate it