The duping situation has both blessed and burned me. Tonight, during a nasty bout of insomnia, I decided to clear some space in my inventory when, lo and behold, items duped. I thought, I may as well try and replicate it. And off I went, to much success. Here's the steps:
1) Have the items you want to dupe in the shared stash.
2) Use the controller shortcut to move them to your inventory, do not pick them up.
2.5) When moving the desired items, try to include throwaway runewords--I had a Stealth and Lore in there. From what I'd seen on YouTube, items with large descriptions act funny with the memory.
3) When you desired items have been moved to you inventory, cancel out of the stash menu and walk down a few paces, basically near Cain.
4) Save and quit.
5) When in your characters select menu, scroll through your characters while counting to ten. Then, exit the game and close the application.
6) Boot back in, check the character and the items should be in the inventory and still in the shared stash.
7) Proceed to bask in your splendor like a demonic treasure goblin.

Keep in mind, this was all done on Switch. My stashes were all full. And after a certain point, I could move just runes to dupe, without the runewords, etc.
It worked with runes, uniques, charms, rings, bases, runewords, everything. And so far, FINGERS CROSSED, nothing has disappeared.
I'll try to snag a video of it next time I do it and share that too.

good luck. - I take no responsibility for any lost Enigmas, Ber's, etc through your attempts.