modloader : fabric / mod version 1.21-10.0.7
dupelicate items in backpack to another one even if it's not the same type
to reproduce the gitch swap the backpack equipped in the trinket slot with another then open the newly equipped backpack with the key bind and take the items from it then open the removed backpack to take the items again

10.0.7 is old version, the current latest version is 1.21.1-10.1.15 and this bug is fixed

Try updating accessories mod to latest version for 1.21.1 too

I can't find that version in tlauncher and when I use version 1.21.1-10.1.13 it says not compatible with accessories mod

I am using version 1.21 not 1.21.1 for my server bc some mods don't have a 1.21.1 version

1.21 & 1.21.1 versions are marked as compatible, it means you can use 1.21 mods on 1.21.1 version if I'm not wrong

I downloaded the latest version possible of every mod on tlauncher except travelers backpack

but have you downloaded the accessories latest for 1.21.1?

I downloaded latest version possible in tlauncher which is 1.0.0-beta.35+1.21

I see, then I can't do much to help you. Like I said 1.21 & 1.21.1 are marked as compatible versions for Fabric, Forge, NeoForge.
This bug is fixed in latest version of Traveler's Backpack. You can try manually adding the mod if launcher doesnt let you download 1.21.1 version.

thank you very much your help I will try adding it manually