So, as you know there is a bug with the shared stash that items dissapears in offline. This is because the shared stash does not save when doing a save and exit but rather in some interval or random. This bug can be used the other way around to dupe
First place the item you want to dupe in the shared stash, play for a while doing multiple save and exits to make sure the shared stash is saved. Close down the game and boot it up again, enter a game and open the shared stash.
Now, locate the item to dupe, move it to your inventory and immediately do a save and exit and then close the game. Now the item is saved to your inventory and MAY still be in the shared stash. Restart the game and see if it worked, if the shared stash was not saved it will be in both your inventory and the shared stash, otherwise only in your inventory.
And again, this is for single player(offline) only. The bug doesn’t exist online.