If a batter who reaches first on an error gets a double how is that scored?
If he gets on first due to an error (like a missed or wild throw to first) and the batter makes it to second it is still just an error.
A batter who reaches first on an error gets charged with a time at bat and credited with nothing. If they reach second, then they might be credited with a single and advancing on an error, or it might be ruled a two-base error.
It is scored a 2 base error if the batter reached 2nd base on the same at bat as the error allowing him to reach 1st base.
It's an error all the way (a two-base one, at that) because it allowed the batter turned base runner to reach base to begin with. No hit would be credited.
It's only recorded as a "double" if you get a legitimate hit and reach 2nd base. If there is an error, it doesn't matter how many bases you run, it's still scored as an error. (E6, E7, etc.)