I've confirmed this exists for beta builds 0.8.0 b5 through b8, at least.
I'm on Android 4.4, Nexus 5.
Hotbar Displays Duplicate Items When Hotbar Item Breaks
It's important to note that the item itself does not seem to duplicate. The same item occupies more than one slot on the hotbar. When you look at inventory, like when choosing what to put in a chest, the correct quantity of items is shown.
I'll just look down and see at least one slot containing another of the same item. When the item is a tool, the same wear is also shown on the copied item.
If I change out the items in my hot bar, the problem goes away. Until it happens again later.
Steps to duplicate:
Obtain up to 6 unique items types, but no more, with at least one of them being a tool.
Put all 6 items in your hot bar, with the tool occupying the first, leftmost slot.
Break that tool from normal wear.
Observe the rightmost box populate with a duplicate item already in your hotbar.