Well, it's pretty risky, but I know that it works in my Silver version so...
1. Have the Pokemon you want cloned in your team, and access Bill's PC.
2. Change to a different box (thus causing the game to be saved).
3. Deposit the Pokemon in that box.
4. Change to a different box, and when it's saving, right after the word "POWER" in "SAVING GAME DON'T TURN OFF THE POWER" comes up, shut off your Game Boy.
5. Turn on your Game Boy. You should have the Pokemon in your team, and a clone of it in the box.
Good luck! I dunno if it works with Crystal, though.
(Oh yeah, and you can multiply items this way also. Just make the Pokemon hold the item.)