Season 6 Episode 5 - Nightwalkers

Carter, Teal'c and Jonas investigate the death of a scientist named Fleming who was working with Immunotech formerly owned by Adrian Wright (from season five) contacts Sam, and find a small town whose inhabitants are part of a secret experiment. The inhabitants are acting mysteriously. It turns out that the experiment involved cloning of Gou'ald symbiotes and all of the townsfolk are infected with weakened clones who can take over the townspeople when they're asleep. Meanwhile the NID are watching the town to take the ship that the Gou'ald are building for the benefit of Earth's technology. The townfolk have infiltrated the NID team and everyone gets captured. Sam manages to use a cure that Fleming developed to prevent herself from being taken over, and manges to undermine the clone-Gou'ald, stop the plan to infect the government with the clones, and keep Teal'c and Jonas uninfected as well.