The only ways to get more than one of a Pokemon are as follows:
1. Catch another
2. Put it in the daycare with another of the same Pokemon of the opposite gender and wait for an egg
3. Put it in the daycare with a Ditto and wait for an egg
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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.
no
You can't duplicate items unless you use a gameshark