Have at least two pokemon with you.  Now deposit one of them into a box on the PC.
Now change to an EMPTY box on the PC.
When it says SAVING...DONT TURN OFF THE POWER, turn off the power. Now turn the game back on, and look into your party. You should have the pokemon that you deposited into the PC with you.
Now look into the PC. The pokemon that you deposited will be ni there also. You can withdraw that pokemon, and you ahve duplicated a pokemon!
To duplicate an item, duplicate a pokemon that is holding the item you want to duplicate.
This can also be used to get all of the starter.
You can also duplicate 2 pokemon at a time.