Ideally it is not advised to work the same muscle group more than once a day. In addition, after working any given muscle group the rule of thumb is to provide at least 1 days rest. Now many bodybuilders will work 2 or more muscles in one day. For example if your working biceps and triceps, you could do biceps in the morning, and then do triceps in the evening. Or chest and legs for another example. Do legs in the morning, and chest in the evening... You will find many different methods and combinations. But again, the rule of thumb is to allow at least 1 day rest for each muscle group. In addition it is advised to take at least one day off a week to allow for recovery and rest.