Using a beacon allows inventory desync

Using a beacon allows for creation of ghost items. If you add a power to a beacon, you are able to visually duplicate items by pressing the swap items key. The item left in the offhand is a client-only item that will vanish if clicked in survival mode, but can be picked up in creative mode.

What I expected to happen was...:
Pressing the swap items key world move the item from my offhand to my mainhand.

What actually happened was...:
It duplicated the item to my mainhand, leaving a ghost item behind in the off-hand.

Steps to Reproduce:
1. Build a beacon of any level.
2. Put the item to duplicate in your offhand.
3. Have nothing in your mainhand.
4. Open the beacon interface and put an iron ingot (or any other power item) in the empty slot
5. Select a power and click done.
6. Press the swap items key.