Depends on what game you're on. And aren't there other ways of getting different Pokémon than catchable in your game than trading over platinum? Like importing from Ruby/Sapphire/Emerald
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That's impossible.
Try the GTS trick.
No. The only way to move Pokemon from Platinum is through a trade. 1 for 1.
The Trophy Garden is the only place to get Ditto in Pokemon Platinum, without trading or cheating of course.
Then you're only hope is trading for a Charmander.