In the newest version 5.3.5 for 1.16.4 the dupe bug is still around, dupped netherite ingot, gold blocks and more to confirm,
this was happening within the backpacks 3x3 crafting grid, the items disappear when you craft the item but come back when you reopen the backpack

In which form did you use the backpack?

it was on my back using the hot key to open the inventory

https://youtu.be/U_zWga5QtSA heres a link to show it in action, sorry for the shaking.

https://www.curseforge.com/minecraft/mc-mods/travelers-backpack/files/3165421