need help with my fathers wrongful 25 yr stacked sentence appeal HABEAS CORPUS Sentencing, Plea Bargains

please anyone who can help offer me some advice.....

My father recieved 2 ten year sentences for arson and mail fraud.
I'm not sure exactly how it works but somehow they sentenced him 2 times for the same thing, I think its called stacked sentencing?
Hes trying to appeal one of the sentences.
He tried a few appeals but none of them came through so now hes shooting for the habeas corpus appeal, which if im correct is usually the last option someone has after they've exhausted all the other appeals?
I cant believe this happened, and that he received such a harsh sentence when he had a family with 5 children to leave behind.
they never told him when the trial began that he could be looking at that much time...
Can anyone give me any details about habeas corpus appeals or offer me any other advice, im glad to take it.
i really really need it this is incredibly heartbreaking for my family and i know that theres some way to get his sentence reduced if its done right.
I'd like to add that he has absolutely NO prior convictions or criminal record at all except for a few traffic tickets.