Goodbye Gemini (also known as Twinsanity) is a 1970 British psychological horror film directed by Alan Gibson and starring Judy Geeson, Michael Redgrave, and Martin Potter. Based on the novel Ask Agamemnon by Jenni Hall, it concerns a pair of unusually close fraternal twins, Jacki and Julian, discovering Swinging London while home on Spring Break. Their experiences complicate the pair's relationship, which is already strained due to Julian's incestuous fascination with his sister, which he sees as a natural manifestation of what he believes to be the pair's hive-minded nature.

The film was produced at a time when conservative groups were beginning to react to the perceived social excesses of 1960s British culture. It was released concurrently with Freddie Francis' Mumsy, Nanny, Sonny and Girly (1970), another horror film which also dealt with an unusual familial relationship and contained a scene implying consensual brother-sister incest. Both films were targeted by the conservative press as endemic of everything wrong with contemporary British culture, resulting in protests and theaters refusing to show the films.