What I expected to happen was...:
Items will not duplicate

What actually happened was...:
Sign that I was holding duplicated when i exited the inventory and put the sign in my hot bar, and when I used the sign in my hot bar, it said sign as the name, but actually when used is a stone block. The duplicated sign was a sign though, and it wouldn't stack on the hotbar sign

Steps to Reproduce:
1. Put blocks on hotbar while escaping the inventory menu
2. You should see the object fly out, but yet have a copy of it in the hotbar.
3. Use the object.

Note: This has only happened this once.