A three-alarm blaze inside a six-family duplex in Mattapan Saturday night left two residents and two firefighters hospitalized, and displaced 20 people, the Boston Fire Department said. The three-story building at 4-8 Theodore St. had heavy fire showing from the rear on floors two and three when firefighters arrived around 10 p.m., said Steve MacDonald, a spokesman for the fire department. Two residents of the home were transported to area hospitals, one suffering from burns, he said. The other resident had an unknown injury. Two firefighters were also hospitalized when a chimney collapsed on them, one with a broken wrist and the other with a broken arm, MacDonald said.