NOTE: This requires at least two pokemon, and access to a PC.
First, take the pokemon you want to duplicate, put it into your group. Make sure you have a box with at least one free slot. Save your game in front of your PC. Then, go to the box, and deposit the pokemon. Exit the box, go to ''Change Box,'' select any box, and while it's saving, turn off the Gameboy.
NOTE: Make sure that you do it while the message ''Saving, do not turn off the power'' is still there. Otherwise it will not work.
Turn on the Gameboy, the box should be the one that you deposited the pokemon in. You should have the pokemon in your group and have the same pokemon in your group.