Under some conditions items can duplicate into offhand without updating the GUI

I was improving a village in creative mode when it placed a door from my first hot-bar slot instead of the block I had selected. I then figured out through trial and error that the door was placed as if it were in my offhand (see 2015-08-10_21.18.53.png). Upon reloading the world for further testing I noticed that the door in question had been duplicated into my offhand which explains why it placed the door instead but I have no idea how or why this happened.