Things You'll Need
Instructions
Connect the two Game Boy systems with the Game Boy link cable. You'll find the port on the right side of your Game Boy, just above the volume controls.
Start both "Pokémon" games and head to the nearest Pokémon Center. Approach the lady at the counter on the right and select "Trade." Note that Player 1 should have the Pokémon you wish to duplicate while Player 2 should have a weak Pokémon you don't mind losing.
Initiate the trade. Player 1's Pokémon will travel across the link cable to Player 2's Game Boy. Turn off Player 1's Game Boy right before it says "Trade Complete." Player 2's Game Boy will say "Trade Complete," and both games will now have a copy of the duplicated Pokémon while the other, weak Pokémon will be lost.