Move items from one slot (in the hotbar) to another using keys (1-9).

I moved an item from a slot to another and then when I pressed right-click to place the block, it duplicated it.

1. Have a block (something placeable) in your hotbar, then a different block (again something placeable).
2. Move it from a slot to another using keys

{1-9} (e.g. from slot 1 to slot 8, it doesn't matter).
3. Place the block you moved.
4. It will "convert" to the block you PLACED, not moved using the keys{1-9}

. What I mean is, if you have Smooth Stone in your selected slot and place it on Glowstone, it will convert into glowstone.