Provided you have had adequate sleep / energylevels , hitting the same muscle twice a day can be beneficial for some people, provided they carefully watch the total workout volume and do their strength related work first. Example chest:
session 1-3
Morning:
5 * 5 flat benchpress
5 * 8 incline benchpress
Evening:
3 * 10 low incline dumbell press
3 * 10 incline flyes supersetted with cablecross
3 * 10 flat dumbell press
3 * 10 flat flyes supersetted with dips
3 * 10 partial pullover supersetted with machine press
....This will do a 4-5 days damage at least before you hit chest again
session 4-6
Morning:
5 * 5 incline benchpress
5 * 8 flat benchpress
Evening:
3 * 10 incline dumbell press
3 * 10 low incline flyes supersetted with cablecross
3 * 10 decline dumbell press
3 * 10 flat flyes supersetted with dips
3 * 10 partial pullover supersetted with machine press
This twice a day stuff is certainly not for everybody and most people can only get away with it if they restrict it to one or two musclegroups they feel like giving extra attention....