To my knowledge, the primary muscles of the back are the rhomboids (middle back), lats, and traps. I currently work each of theses muscles twice per week. Therefore, my back workout looks something like this:
Barbell bent-over rows
Dumbbell rows
pull ups
lat pulldowns
barbell shrugs
dumbbell shrugs
Is this overwoking my back, since many of the middle back and lat exercises hit similar muscles?