Problem with doubling up math is that some other course gets squeezed out of the schedule. This can be especially a problem at 6-period high schools, where getting the basic “4 years of each of English, math, social studies, science, foreign language plus a year of art” along with other high school requirements like PE and health is already a tight fit. This might be a reason why parents push for math acceleration in middle school.
Of course, overacceleration happens also, as evidenced by threads about students in the +2 math track struggling when they get to precalculus or calculus.