The number of people in San Francisco who got acquired immune deficiency syndrome through injecting drugs has almost doubled in six months, prompting one doctor to warn that the city is "on the verge of a second epidemic." Chris Geoghegan, a disease control investigator with the San Francisco Health Department, said there were 12 cases of AIDS contracted through intravenous drug abuse in the city in December, 1985. As of June, 1986, there were 23, she said. Another 232 cases have been among people who are both drug abusers and homosexuals, said Dr. Dean Echenberg, head of disease control for the Health Department. Homosexuals and intravenous drug users are two groups hit hardest by the disease, which robs the body of its immune system. Dr. Richard Chaisson of San Francisco General Hospital said, "This year, with IV drug users, we're where we were in 1981 with gay men."