I use 1.12.2 version find a serious bug, player can copy item
First you need wear a pack
Second you open a pack on you hand
Third throw away the pack on you hand
And then take the item on the GUI

#73 (comment) They are the same bug. @Tiviacz1337 You can charge "Travellers-Backpack\src\main\java\com\tiviacz\travellersbackpack\gui\container\ContainerTravellersBackpack.java:
if(x == currentItemIndex && !this.inv.hasTileEntity() && !CapabilityUtils.isWearingBackpack(this.playerInv.player))“ to
”if(x == currentItemIndex && !this.inv.hasTileEntity())” to fix it. I tested it. It works

I have tested and confirmed this, it's such a serious bug. Players can get any amount of items as they want, with just two backpacks. Please fix this first as soon as possible, thanks. @Tiviacz1337