Saving Problems [Pokémon Platinum]

I've just recently bought Pokémon Platinum pre-owned and subsequently there is saved data on the game, but I didn't realise I had to delete the saved data before saving, now it won't let me overwrite the save game. Any help? P.S. I don't want to have to start again, oh and I'm on DSi if that helps :) Thanks

Compare the back of the DS game with another DS game. If the Nintendo logo feels a bit more bulkier, that's your problem and it is likely to be a fake. If not, I have no idea.