How do you duplicate Pokemon in Pokemon Pokemon LeafGreen?

you can only duplicate items in the game by going through this process:

1) Enter a Double Battle in your game.

2) Send out a Ditto with no item and a Pokemon with the item you want to duplicate.

3) Have Ditto use Transform on the Pokemon with the item.

4) Switch the Pokemon that was stolen from with a Pokemon that can use Thief.

5) Use Thief on your own Ditto.

6) Win the battle.

Now the player obviously has two of that item.