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Duplication is the process of multiplying already obtained items. It simply doubles the amount you already have.
For best results, place the intended item at the top-left/right corner of your inventory. Click it and throw it away. Alternately, you can equip it and press 'Q' to drop it. Press 'Esc' and click on "Save and Quit to Title". Re-load your world, pick up the dropped items and immediately press 'Esc'. DO NOT save & quit, instead, wait for the text "Saving Level" to disappear and then completely close Minecraft by clicking the red cross in the top right corner of the window. Remember: Do not click save & quit. Open Minecraft and load your world. Check your inventory and congratulations! You have doubled your items.
You can multiply a pile of 64 items, it just creates an extra pile.
When I did this, it may have been my crummy computer, but it started lagging after just a few dupes. Eventually it stopped, but still, beware.
There is another technique for the Xbox 360 edition as well, using a furnace. First, place a furnace and put an item in it. You must mine it with a pickaxe then switch to your hands at about 95% mined. Then quickly go into the furnace and hold A on the item. 2 items should drop, and drop the one you can pick up to pick up the other. Finally, hold X to start duplicating!!! (Please note this takes practice and will usually not work on the first try. There is also a chance to lose the item, so be careful!)