Describe the bug
You're able to duplicate items using the backpacks and a Modular router with a player module.

You can set the player module to only pull from main inventory excluding the hotbar. You can then have the bag in your off-hand, open it, and then hit 'F' on one of your inventory slots to put it in your inventory. This removes the bag but you can still access the inventory and pull stuff out. When you close the inventory and grab the bag from the modular router, the backpack inventory is untouched even though you took items out.

Note
I understand that this could be fixed, but would require a rewrite of the mod, as the issue lies in the fact that the storage data is held in nbt on the backpacks.

Versions
Forge: 32.0.108
Simply Backpacks: 1.4.10
Modular Routers: 7.0.1-29

Fixed in d017ce5 just havnt uploaded it to curse yet.