Boomer/Athena – Battlestar Galactica, “Someone to Watch Over Me”
Part of the fun of there being multiple Cylons within the same model was when they would differ so markedly; in BSG, you had a “good” Sharon (known as Athena) and a “bad” Sharon (Boomer). Of course, it’s the evil one who’s more interesting: In season 4, Boomer returns to Galactica a traitor and beats up Athena, gags her, and hides her in the closet.
Boomer gets dressed in a pilot’s uniform ready to continue with her plan, but then Athena’s husband Helo comes home. Thinking this is his wife who’s about to go on a six-day mission, he starts to seduce her. Boomer initially refuses his advances… then suddenly decides to go with it. The best part is when she makes eye contact with poor Athena in the closet before proceeding to have hot Cylon/human sex with Helo.
Madelyne Pryor/Jean Gray, X-Men
In Madelyne’s defense, when Scott Summers left behind his Cyclops identity to marry her, everyone thought she was a normal, non-mutant human. It turns out that family life in Alaska got too boring for Scott, so when he found out that his love Jean Gray was mysteriously alive, he abandoned his wife and son. Boy, was everyone in for a doozy: After Madelyne joined up with the X-Men to find her kidnapped child, she learned that she was a clone of Jean Gray, created for the express purpose of conceiving a child with Cyclops.
Not surprisingly, Maddie went crazy and became a supervillain known alternatively as the Goblin Queen, Black Rook, and Red Queen.