This is a clone glitch for Pokémon Emerald that doesn't require Action Replay, Gameshark, or any other cheating device. All you need is Pokémon Emerald!
If you have Emerald, follow these steps, and you'll be making a fortune!
Go to the Battle Tower in the Battle Frontier. (you must have beaten the Elite Four first)
Go to the PC and deposit some Pokémon to make free space, then save the game.
After you've saved the game, withdraw the Pokémon you want to clone. You can clone five Pokémon at a time if you made enough space in your party. WARNING: Do NOT replace another Pokémon position in the party! If you do that, the Pokémon will be lost! To make sure that you don't screw up, just choose WITHDRAW POKéMON in this step.
When you've withdrawn the Pokémon you want to clone, go down and talk to the first lady on the left. Choose either Lv. 50 or Open Level, then Submit two random Pokémon. Then the lady will tell you this: Before entering a BATTLE ROOM, your progress must be saved. Is that okay? Choose Yes. Now the screen will freeze for two seconds. When the save screen comes up, turn off the game.
Turn on the game and check your PC. If you did it correctly, you will now have the same Pokémon in your party and box!
Here's a video of someone doing the clone glitch. I suggest watching it so you don't screw up.
This clone glitch can be exploited in many ways to give you advantage. For example, you can use it to clone Rayquaza to beat the Elite Four, or you can make the Pokémon hold an item to clone the item too! You can clone Master Balls, Nuggets, Heart Scales, TMs, Sacred Ashes, and much, much more! You can also use it to supply your Pokémon DS games with items and Pokémon using Pal Park. I did that with my Heart Gold, and now I have 38 Master Balls and 30 Heart Scales.
The best thing about it is that it's not a cheat or code like Action Replay that could damage your game. This is simply a glitch that can do no harm. As long as you do it right and don't lose your Pokémon, you won't have a problem with it!