fabric 1.20.1-9.1.18
when you use the /give command to obtain a backpack, you receive two backpacks. I am a modpack author, and I tried to place rewards inside the backpack. However, when receiving the backpack reward through FTB quests, I also end up getting two backpacks.
I don't know if there are other ways to obtain double backpacks, but I feel this bug is very dangerous.

Unfortunately this is more complex bug, it's happening even on Forge/Neoforge and has to do something with how minecraft handles /give command. I suggest giving empty backpack instead, that will give only 1 backpack