Run, Psycho, Run (1968)

Italy, the early 19th century. A villa is the home of a small English community, all rich and superficial people lounging about in its beautiful surroundings. Claire, George's wife, and Robert, her son, are murdered by a mysterious killer. Thinking the murderer is one of the family, distraught George devises a plan to unmask the culprit. Having met a woman, Ann, who is a dead ringer for Claire, he takes her back to the villa and announces their imminent marriage. Ann is considered an outsider and treated with contempt and hostility, while George fails to find any significant clue to the solution: everybody is to be blamed, at least morally, but the final revelation is more bitter than expected...