Caught a strong Pokémon and want another one of the same kind? Cloning is exactly the way you need it. Take advantage of a bug in the game to become the most powerful trainer in the world! You can easily create a copy of a Pokémon with all the items it holds, all without having to resort to tools like Gameshark or ActionReplay.
Steps

Step 1. Move Pokémon to PC to free at least 2 crates

Step 2. Add the Pokemon you want to duplicate to your group

Step 3. Give the Pokemon a valuable item, a copy of which you would not mind getting (masterball, candy, ingot, etc. in the same spirit)

Step 4. Switch to an empty box on your PC
When asked if you want to save, select Yes.

Step 5. Leave the Pokemon you want to duplicate in an empty box on your computer

Step 6. Now switch to another empty box on your PC
Note: When asked again if you want to save, select Yes, but keep your eyes on the message below.

Step 7. At the bottom of the screen, the caption will blink
"Saving in progress, do not turn off the power." When the word "power" appears on the screen, turn off the power.

Step 8. Turn on the power
Check the group and PC. If everything worked out, the Pokémon will not only remain in your group, but will also appear on the PC.
Advice
- Clone 6 Evie to get all 5 of Evie's evolutions in addition to Evie himself.
- With this method, you can clone any Pokemon and item.
- Do you want to know how it works? When changes are made to the game data, it first saves the Pokémon data in the drawer and then deletes the Pokémon data in the group. But if you unplug the Game Boy in between these two processes, the Pokémon will remain not only in the group, but also on the PC.
- Give your Pokémon rare or valuable items to get the most out of the process.
- Trade a cloned Pokemon (preferably rare) for one you don't have.
- Practice, practice, practice. If something goes wrong, release the buggy Pokemon and try again.
- This trick works in Pokémon Crystal as well, but it is much more difficult and risky to perform.
Warnings
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Read all instructions again before trying again!