On 1.16.5 latest version it seems have two backpacks of the same grade on hotbar with stuff in one and not the other you can dupe so they have the same items. Let’s say backpack with stuff is slot 2 and with no stuff slot 3. If you hover over backpack in #3 with #2 open and press 2 it’ll dupe the items in #2 to the #3 backpack you tried moving. Sorry if that was confusing to explain.

#128 Duplication with the backpack but another way

Getting on fixing this one :)