A fire tore through a duplex home in Lynn Saturday night, causing moderate damage to the structure, Lynn fire officials said. Firefighters arrived at 67 Marion St. after 9 p.m. and the fire was out by 10:21 p.m., District Chief Robert Bourgeois said. No injuries were reported, Bourgeois said, but a cat was killed. The cause is still under investigation, but Bourgeois said that it appears that the fire was caused by an electrical fan in the basement. Kat Powers, director of communications at the American Red Cross of Eastern Massachusetts, said the agency is offering lodging and emergency funds for the six people affected by the fire. “These folks did lose a significant portion of their belongings,” she said.