I am able to have two curse of binding books on top of each other

What I expected to happen was...:
I would have 1 curse of binding book

What actually happened was...:
I had 2 curse of binding books on top of each other

Steps to Reproduce:
1. Use a Grindstone
2. Put in a curse of binding book on the top on the left
3. Put in a curse o binding book in the bottom slot on the left

4. You will see 2 curse of binding books.

I have attached pictures