Hey, it seems that it's still possible to duplicate items using the number hotkeys:

Here's what I did:

• Have two stacks in the bag
• Use the number keys to move one stack to the slot where the bag is, moving the bag into another slot
• Grab the bag and shift-click on the other stack so it lands in the inventory
• Throw the bag out of the gui

When you now pick up the bag it still contains a stack.

Version used: 2.1.5b

Thanks for pointing this out. We're still having trouble finding a solution in order to prevent this happening. Hopefully I will have one soon :)