If you open a Dank and drop it at the same time, you can remove the items on the GUI while the dank on the ground will still have them.

This is not possible, the dank immediately closes once the item in the hand no longer matches the item when opening the gui.

You are wrong.
https://www.youtube.com/watch?v=PuKoTXPjPOk

I forgot to pick it back up and open it in the video but it still has the item inside it. I can upload another video if you'd like.

Now do it in an instance with 0 other mods, I will only reopen this if it doable on the latest version (3.16 as of this comment).

Simply join a server that is NOT hosted on the local network. Having ONLY dank storage mod but hosting the server externally allows for replication of this dupe.

So what I've been getting from other modders is that this is more likely to be desync than actual duping. Given the nature of this bug and the inability to get a reproduction of actual duping rather than desync, this will remain closed.

I don't understand what you mean. You can't replicate this on your own? I have a server hosted on a dedicated machine and my ping is sub 60ms. The server only has the newest version of forge and dank storage and the dupe works. I don't know what you mean by it's desync rather than actual duping, the items you dupe are real and can be crafted with.

If you need a server to test it with I can provide you with one