Except with Downs there's only the one extra chromosome.
Just imagine everything that could go wrong if you had only 26 extra chromosomes, let along the full package.
Then again, you never know, if you had the full package twice it could help to combat certain diseases. If any of the chromosomes are faulty then the second set of 42 (acting as a backup) can be used to replace it. The same way that if one of the X chromosomes in a female is faulty the other is used as a backup instead. It's just bad luck if both are faulty.