You can use a Dispenser or a Furnace (both will work just fine) a stone pickaxe and a stackable item.

1. Jump and place the Furnace/Dispenser under you.

2. Mine the furnace/dispenser most of the way with your stone pickaxe (or better) for a few seconds Then before it is destroyed switch to holding something else.(not a pickaxe)

3. Dispenser: while still standing on the dispenser place one of any stackable item in the dispenser, and hold down A or "place all" until the dispenser dissapears.

4. Furnace: While standing on the furnace place any one stackable item in the Fuel section. And hold down A or "place all"

If successful there should be one of that item in your inventory, and one floating on the ground. If so, drag the one item into your inventory and hold X or "place one" to make the block real and moveable.

EXTRA: If you successfully duplicate one stack, then you can place the glitched block in your hot-select to make a never ending stack of that item or block.

No duplication glitch will work every time. Sometimes it may take that block, or just the dispenser/furnace. The best part about this glitch is that it is single-player so you do not need a friend to cooperate.