Like most everyone, I don't do the SAME workout twice in one day. That'd put me at risk for overuse injuries. If I want to work out twice, I'd do cardio in the morning and weight work (or core work) in the afternoon, or lower body in the morning and upper body in the afternoon. Never the same body part twice on the same day. Also, I don't double up on my workouts all the time. That is very stressful on the body, even if I double up only three days a week. The fatigue eventually catches up. When it does, I get diminishing returns from my workouts. There are only certain times of the year when I do this, and even then, I don't stretch the doubling-up period further than three weeks.
Pinky