I was using a diamond shovel to dig dirt. In my inventory I had a diamond pickaxe, not equipped. I had no other shovel or pickaxe. Suddenly the sound changed and I was using a diamond pickaxe. I immediately stopped using it. Then to my surprise in my inventory there were TWO diamond pickaxes, with identical durability and no diamond shovel. I was shocked, switched tools, and experimented a bit, but did not regain the diamond shovel. I am quite sure the two pickaxes showed up for quite some time. Finally I returned to my base in order to craft a new diamond shovel. Guess what? As soon as I started crafting, my inventory showed ONE diamond pickaxe and ONE diamond shovel with more or less the expected durabilities.