Sometimes, but I never do high intensity stuff with tired or sore muscles. I much prefer 1 high intensity workout followed by a full rest (of those muscle groups) to 2-a-days, though sometimes I like to mix it up just to keep it interresting. Depending on how intense your workouts are, I see no problem of working out twice in a day with plenty of nutrition and rested muscles.
Even when I do 2-a-days during a week, I never seem to go above 4-5 workouts a week total. The rest period is when you're making all the gains! Don't overdo it.
Like SC, I've also been in a rut (more than once actually) from working out too much and staying injured. I've learned my lesson there. Trust me, you'll stay more consistent and make better gains if you give your body plenty of rest. Of course, as you improve, you're able to do more, but the basic rules are still the same.
I will say that oddly enough, I didn't really learn to listen to my body (as far as when and how much to rest) until I started really paying attention to my nutrition and caloric intake.