Got an odd problem, currently working on how to replicate, but I had two backpacks in my inventory that I named on an anvil, one for tools, and another for dungeons, and whilst dungeoneering, noticed that suddenly I had two tool bags! (Not complaining, there was a lot of valuable stuff in my tool bag, lol), but still,that happened, will edit if I can reliably replicate.
Okay, it appears to happen when I have a backpack equipped in my camping mod inv slot. Any other bag I open in my hand appears to have a chance of becoming a copy of the one I have equipped. It also does this quite randomly. A couple times in testing, it reverted back to the original, only to change back to the copy again the next time I opened my inventory.