Describe the bugbes
You can dupe entire backpacks and the items inside them with 2 combined Backpacks
To Reproduce
Steps to reproduce the behavior:

1. get a large backpack and two(2) combined backpacks
2. combine one (1) of the combined backpacks with the large backpack
3. now combine the both of the combined backpacks, the empty combined backpack needs to be in the middle of and the combined backpack with the large backpack inside on the right middle field of the crafting bench
4. if you open the combined backpack within the combined backpack, you can just pull the backpack out and put it in your inventory close the overlay and repeat.

Expected behavior
You either shouldn't be able at all to combine two (2) combined backpacks with each other, or you should be able to open the large backpack inside of the 2nd combined backpack within the combined backpack.

Screenshots
https://imgur.com/a/AE1BiRG
Version info
CraftBukkit version git-Bukkit-ba575a5 (MC:1.14.2)

Looking into it

Ended up having time to look at it today. The issue stems from the backpack inventory "tagging" and opening/closing system. I've been meaning to work on it for a while now but haven't had time because of school.

As a temporary fix I am just going to disable putting combined backpacks in other combined backpacks. Thanks for the report!

Fixed as of 2019.1.6! see commit