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.
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note the duplicated bow in the hotbar after the left-most item broke and all of the hotbar items shifted over to the left one.
Duplicate of MCPE-6123
Confirmed this still happens in 0.8.0 beta build 8.
I've narrowed down the behavior to when an item breaks, the hotbar moves all of the items over to the left one box, and auto-fills in the right-most box with another item from your inventory. If your inventory doesn't have many items in it, it will display a copy of the same item already in your hotbar.
Expected behavior: I expected either analgous behavior to the PC version (ie. the box containing the broken item remains empty after it breaks), or, if the auto-shifting/auto-filling behavior is intended for PE, then I expected the right-most box to remain empty if your inventory doesn't contain enough items to autofill the hotbar.
Having two of the same item in the hotbar just to keep it populated seems unnecessary, as is just confusing to see two of the same item.
See screenshot for example.