Valerie,
I don't have time to go into detail now but I will. About Nala... who is the aggressor / instigator? Does Duke look at Nala in a certain way as to provoke her or warn her to stay away from him? Your last post certainly did shed light on the fact that Duke is in pain of some sort. I would take him to the vet if the meds he is on are not working. More later.