Goodbye Gemini (1970)

Unnaturally-close twin siblings Jacki (Judy Geeson) and Julian (Martin Potter) become enmeshed in the swinging London scene, where they attract the attention of heavily-indebted gambler Clive (Alexis Kanner). Seeing a way out of his debts, Clive sets Julian up for a blackmail scheme, setting in motion a nightmarish sequence of events.