Whenever you break a tool or weapon, and the inventory hotbar moves the items, a duplicate of a tool or block will appear in the last slot. Like the hotbar will show, for example, two shovels, when really you're just carrying one.

What I expected to happen was...:
Nothing

What actually happened was...:
An item was cloned so now I have two shovels, but really just one.

Steps to Reproduce:
It's actually hard to explain, but playing Survival mode long enough and at least breaking some weapons or tools will show the glitch.

The attachment shows the two shovels in the hotbar below, but in my full inventory, I only have one.