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.
Chat with our AI personalities
First of all , You need a Ditto ( That you can catch in Pokemon Mansion , In Cinabar Island ) .
Then , Talk the Ditto and the Pokemon you want to Duplicate , into the Day Care Center ( Situated , south of Cerulean City ) .
Then leave the Day Care Center , then Come back in , The Guy will give you an Egg , This Egg is the Duplicated Pokemon .
Hope i helped ( If i did , Please Recommend me and my Question )
You cant do it in leaf green, only in Emerald....
Yes you can trade leafgreen version Pokemon to another leafgreen version.
Im sorry you can't duplicate Pokemon in Pokemon platinum. You can duplicate Pokemon in diamond/Pearl or emerald the trade or migrate them to platinum though.
You can't duplicate items in Pokemon. Only if you have the Action Replay or GameShark, then you can.
As far as the game goes, you can't duplicate Pokemon on Sapphire. Tried it and it glitched up.