I had two stone hoes and they were both at 51 durability and in the grindstone i merged them but i got 108 durability on the stone hoe, and I'm pretty sure 51+51 isn't 108. Idk if this is a feature or a mistake in the programing? I thought it was worth while to note.
Was always like that when repairing in your inventory before the grindstone block was added into the game.