How to clone items and Pokemon!
Put the Pokemon in a empty box what you want to clone, and make that Pokemon hold an item you want to clone, e.g masterball, rare candy etc. Then once you've put the Pokemon in the empty box, change box, when it says saving do not turn off the power. Switch it off. Then switch it on and you will have one of that Pokemon in your party and one in the box and one of the items will be in your pack, and the cloned Pokemon (in your box) will be holding one.
We have more cheats and tips for this game here, Pokemon Emerald cheats