While making a video a pair of diamond boots in the 3rd row 4th column of my inventory duplicated while putting granite in the hotbar slot directly under it, when I released the mouse button the diamond boots took the place of the granite being held by the cursor and as I pressed Esc to exit the inventory the boots left my mouse cursor and flew into the air which I then picked up.

So far I have only been able to create 2 granit while trying reproduce this glitch, however I'm unable to explain how I did it other than, I sat there redoing the same movements from the attached gif.