You can either follow the in-game tutorial here ------> http://www.planetminecraft.com/project/block-duplication-glitch-tutorial/ or you can follow the blog...

NOTE: This may not work in the latest version of minecraft but it worked perfect for me...

You will need:

4x Redstone Dust

2x Repeaters

1x Diamond Block (or block you wish to duplicate)

1x Redstone Torch

64x Dirt or similar value-less block

1x Sticky Piston

STEP 1: Place two redstone repeaters adjacent to eachother, so each one faces a different way. Now place redstone dust around it creating a small circuit. Then place a redstone torch to power the circuit, and then you may delete it.

STEP 2: Now quickly delete and place redstone dust which will create a flasing circuit. Lead this off with more redstone and connect it to a sticky piston.

STEP 3: Place your duplication block of choice and the end of the piston and it will move back and worth. Now take some dirt and quickly place it in the gap between the diamond and the piston, which will glitch out and create a whole new diamond block! You can do this up to a certain point until you have to collect the diamond and start again.

If this worked for you, please post a comment or diamond either of these posts :)

NOTE: I did not discover this glitch but whoever did I give credit to them...