Holding down a key to switch an item to another inventory slot while hovering over it in the inventory sometimes causes the item to duplicate.

For an example, see https://youtu.be/yVnveKIz0n0. I hold down 9 while mousing over the sword. The bug seems to be more consistent at higher render distances (i.e. lower frame rates). It took multiple minutes of holding the button down to trigger the bug at a render distance of 3, for example.

UPDATE: Reproducing the bug on a pure vanilla (non-Optifine) version of Minecraft: https://youtu.be/GClXqYVNqbg

The bug took considerably less time to trigger, further suggesting that it is tied to framerate (Optifine being a performance-enhancing mod).

Also of note is that sometimes it isn't the item that gets duplicated but rather the blank hotbar slot one is swapping it with: https://youtu.be/ZGc7drZPRIE