I was reorganising my storage with a Mekanism Cardboard Box, and put my Dank Dock (containing a Dank 7, which in turn contained a 16x16 block quarry's worth of materials) in it out of habit. When I put it down where I wanted it the contents were gone, and it contained a copy of my own inventory. I then loaded a backup of my world, went to bed and made the exact same mistake the next morning, proving both that the issue is replicable and that I'm a dumbass. I didn't investigate further to figure out whether I'd duplicated my inventory or was accessing my inventory through the dock.

I encountered this while playing FTB University 1.16 v1.8.0

Steps to replicate, that can be sung to the tune of Justin Timberlake and The Lonely Island's classic Christmas song:

1. Put your Dank in a Dock
2. Put your Dock in a Box
3. Then you open the Box

And that's the way you do it! It's a Dock in a Box!

Decided to test this more because someone in discord said something along the lines of "imagine duplicating diamonds".

Shift-right-clicking the box makes the box itself disappear.

Moving an item from or around the glitched dank appears to work as normal at first, as if the dank is a separate inventory containing a duped copy of your inventory. But after you've moved an item to an empty space, clicking another item in the dank that was in the dank when the box was removed (or an empty space where one of those items was at the time) restores the item to the inventory, meaning you can just dupe those items forever.

Closing the dank inventory causes the player to drop the last item they took out, but you can just pick it back up.

Mekanism's Ultimate Logistical transporter couldn't transport items from the dank, but at some point when I set that up the dank inventory updated with my current inventory. Now items moved to the player inventory disappear and return to the dank when it's inventory is closed, unless i put it in the toolbar, in which case it gets dropped and can be picked up, and dank inventory is updated again.

Removing the dank from the dock gives you an empty dank that behaves normally, even when returned to the dock.

Here's the log of this testing session. https://mclo.gs/91EcMZ0

This seems like it is probably related to whatever they did to reimplement a standard inventory to support larger stacks, but don't quote me on that as I'm not a modder.