It's perfectly acceptable to work out twice a day, so long as you're not working out the same muscle group twice. For example, in the morning you could do upper body, and in the evening you could do lower body.
The purpose of strength training is to tear down the muscle fibers and then supplement with protein. The protein breaks down into amino acids and repairs the damaged muscle fibers. Protein synthesis is optimal during sleep, so unless you sleep most of the day between workouts, training the same muscle twice in the same day is actually counter-productive since your goal is to get the amino acids into the muscle for hypertrophy and your body just won't have enough time to allow that to happen without proper rest.
Over-training is bad news. The muscles become fatigued and lose their ability to respond and handle load, and so the forces are placed on the joints and connective tissue.