As a side note from a reader:

Learn how to get multiple starters in Pokémon Platinum by transferring from LeafGreen, Ruby, Sapphire, or Emerald.

To obtain additional starter Pokémon from LeafGreen, Ruby, Sapphire, or Emerald in Pokémon Platinum, you can utilize the same game cartridge multiple times, provided each transfer uses a different ID number.

One method involves playing LeafGreen until you have obtained 6 Pokémon and the Pokédex (Pokedex). After this, you must transfer your Pokémon. To migrate Pokémon, 5 Pokémon plus your starter must be in the PC box. Then, catch Pokémon in the Pal Park in Pokémon Platinum. After the transfer, you can start a new LeafGreen game to obtain another starter.

This process takes approximately half an hour per transfer to reach the Pokédex and catch 6 other Pokémon. If you are not too far into your current game, this is an efficient method to acquire original starters or those from Ruby, Sapphire, and Emerald.