Instructions
Travel to any PokeCenter in the game. PokeCenters are located in nearly every town and are primarily used to heal Pokemon.
Go to the top right of the PokeCenter and interact with the computer by pressing the "A" button.
Store the Pokemon you want to duplicate into any of the boxes on the PC.
Press the "B" button to back out of the PC. As you do so, a screen will appear informing you the game is saving and to not turn off the power.
Turn off the power when the save message appears. The next time you turn on your game, the Pokemon will still be in your inventory and there will be a copy in the PC box.