This is the same cheat as Pokemaster was talking about, just more specific.
This is a very specific procedure, so you'll have to follow it down to the push of a button.
WARNING: first try this on an unimportant Pokemon. Also note, I am not responsible for anything that goes wrong. Also, I have not tried it on Pokemon with ribbons and don't know if that will work. If you mess up the procedure, do not continue on. Leave the building before trying again.
The procedure really isn't as long as it sounds.
1. Fly to the battle frontier
2. Go to the battle tower and access the PC
3. Identify which box in your PC is box 1. (if you named them, there are 14 boxes, so if you named most boxes, use the one after box 14)
4. Completely empty it out. There cannot be ANY Pokemon in it.
5. Make an empty space in your party(do NOT put the Pokemon in box 1)
6. Take the Pokemon you want to clone (item for cloning must be held) and put it in box 1.
7. Close out of the PC.
8. Save the game
9. Open the PC and put the Pokemon you want to clone in the empty slot in your party.
10.Close out of your PC.
11.Walk over to the staff woman nearest to you (she runs the Link Multi Battle Room)
12. Talk to her
13. Accept the challenge
14. Take the Open Level challenge.
15. Enter 2 Pokemon. It doesn't matter which ones, just not the one you want to clone.
16. She'll ask you if it's OK to save. Say yes.
17. It may take a bit longer to load than normal. Don't get freaked out.
18. When it shows you the normal save thing and says 'do you want to save' and shows you the thing with your name, pokedex, badges, and time, turn off the power.
19. When you turn back on the power, (and get into your file) you'll still be in front of the lady.
20. (optional) Walk back to the PC. In box 1, you'll find the Pokemon you cloned if you did this right. There also will be another one in your party.
I'd try it with the instructions in front of you, it's too easy to mess up.