To continue romreacor's post
Yeah, I'll continue romreacor's post. To duplicate a pokemon is pretty simple. First save in front of the PC (to make things a hell of a lot easier), second deposit the pokemon wanted to be duplicated. Next, change box and as soon as it's finished writing:
SAVING....
DON'T TURN OFF THE POWER. Turn off the power.
Our gineepig (sp?) will by Cyndaquil. What happens is that Cyndaquil has been saved in your party in front of the PC and you had deposited him in the PC afterwards. Then you had sort of saved the game but not quite so that a double half save file (to create a whole, 1/2+1/2=1) was merged and loaded. Therefore you've got Cyndaquil in your party, and Cyndaquil in whatever box you deposited him in the PC. Withdrawing Cyndaquil is completely safe, it will not harm your game in anyway.
Have fun having Armies of duplicated pokemon!
NOTE: This cheat doesn't work with every legendairy. I remember Lugia and Ho-Oh not being able to be duplicated, but the starters are duplicatable. You can still try duplicating the legendairies, it won't harm your game as I have tried numerous, numerous times.