Describe the bug
When inserting a a duplicated backpack inside itself with the inception upgrade, the game crashes. I am unfortunately unaware of how the backpacks are duplicated but since they are UUID-based it really doesn't matter. Inserting a backpack inside of a backpack with matching UUIDs will cause a crash.

Crash Report: crash-2021-09-27_18.28.30-server.txt

To Reproduce
Steps to reproduce the behavior:

1. Have two backpacks which share the same UUID. (Unaware how it was duplicated.)
2. Add the Inception upgrade to one of the backpacks (they will sync.)
3. Place one of the backpacks inside the other.
4. Stack overflow.

Expected behavior
When adding a backpack inside another, it should ensure the UUID is not equal. If it is, it should not let the backpack be added to its inventory. Since I am unfamiliar with the codebase, there is probably a much better solution.

Versions
SophisticatedBackpacks - 1.16.5 - 3.2.12.337

this is not a valid error - there's no way in the mod to get two backpacks with the same uuid without switching to creative and duplicating it through it or potentially get it as an item with the same uuid from something like quests. So there's nothing to be done in the mod to fix this.

If you however can recreate the issue in survival feel free to report it including the steps to recreate the issue.

(Both of these are happening on version sophisticatedbackpacks-1.16.5-3.2.12.337, though I am on a modpack and there's a slim chance some other mod is doing something weird and making this happen)

It is in fact possible to recreate this in survival, in fact I came here just to report it. Here's the steps to follow:

1. Grab a backpack, put a Stack Upgrade and inception upgrade into it
2. Make brand-new backpacks, do NOT open them after crafting.
3. Put the new backpacks into the inception+stacking backpack, they will stack
4. Hover over the backpack stack and press the backpack keybind, opening the backpacks and giving them both the same UUID
5. Now you have two identical backpacks

What's more, items can be duplicated easily due to this.

How it works:

1. Place the two linked backpacks after putting some items in them
2. Extract the items, which are now duplicated

Thank you for the fix!