How do you get the TM double hit in Pokemon platinum?

In Pokémon Platinum, Double Hit is not a TM at all, so you can’t get a “TM Double Hit.” If you’re trying to evolve Aipom into Ambipom, level Aipom until it learns Double Hit (it does this at level 32 in Generation IV games), then level it up once more while it still knows the move to trigger evolution.