First off you're going to need a Dispenser, Any Pickaxe, and A map, block, tool, etc. e.g. wooden plank.

Step 1: Place the Dispenser on the ground.

Step 2: Put the item that you would like to dulicate into the dispenser.

Step 3: Pull out your Pickaxe and start hitting the dispenser.

Step 4: Once it is about to break pull your your tool, block, etc.

Step 5: Once it has broke it should re-appear. DO NOT SWITCH BACK TO YOUR PICKAXE!

Step 6: Open the dispenser and up to to the item you're duplicating. Press and Hold A until the menu closes.

Step 7: Your duplicated / glitched block should be on the ground

Step 8: throw away the normal blocks and pick up the glitched one.

Step 9: Start duplicating. Press and Hold X in each slot.

Don't understand this? Here's a YouTube Video.