Well, I wouldn't go so far as to call demodex common. The vast majority of dogs don't get it at all. It is not unusual however.
As for being affected twice... well, what you've got to figure here is that demodex is an immune system problem. All dogs have demodex mites living inside the hair follicules (and you have a human version, btw). They're a natural flora of the coat and normally live in harmony with their host. They cause problems only when the pup has an immune system problem. That might mean an underdeveloped immune system, it might mean an overly stressed immune system (assaulted by multiple vaccinations, operations like neutering, general stresses, etc), or it might mean an outright defective immune system. The fact that he's been affected twice suggests one of the latter two scenarios is the most likely.
What you can do to help him recover and avoid recurrances then, is to boost or support his immune system as far as possible. That means boosting via the highest quality diet you can possibly feed - raw natural diets are generally best, but failing that, a very high quality kibble and supplements as required. And the flip side - removing stresses. That's stress in general, but also specific assaults on the immune system - so no vaccinations or elective surgeries until he's recovered, and think very carefully about chemical treatments like fleas, ticks and worming (all of which have an immune system consequence).
