Stack Duplication

I was testing the new feature that lets you quickly put a stack into your hand by pressing the corresponding number. In my hand I had a door, a redstone lamp and a stack of 64 levers. I put them in the first 3 slots of my hand, and started spamming the 1, 2 and 3 buttons while hovering over the middle slot. Suddenly, I noticed there was more than one door in my hand. I stopped, and the redstone lamp had turned into a second stack of 64 levers, duplicating the first. I never found the redstone lamp. It was completely deleted.