This glitch is a doozy:

1. Have another item next to the bag opener that allows to see the openers storage while not in it. The example here is the workbench in tinkers that shows nearby storages.
2. Have JEI and a materials to craft an item in your inventory.
3. Place the item you want to dupe in the workbench.
4. Find the recipe in JEI and use the + button to add the recipe to the workbench.

This will move the item from the workbench into the upper storage of the opener and dupe the item. Likely due to the block not checking for item ID after it is already in the table since you cannot place any item in it.