Magnum, P.I. - Season 5 Episode 2 : Echoes of the Mind (2) 1988

Conclusion of this two-part story. Higgins must break the news to Agatha that it's not her that he intends to marry, but he finds that Lady Ashley is not the same as the woman that he thought he knew. Meanwhile, Magnum meets Diane's twin sister Diedre, who claims that her sister is a paranoid hysteric who has been in and out of mental institutions, attempted suicide, and has twice tried to kill her. Diane insists that her sister is the bad one and is just trying to bad-mouth her to put Magnum off of her just as she always has done with Diane's men, and as Magnum tries to find out which of the sisters is telling the truth and just what is going on, he encounters one bizarre twist after another about Diane and Diedre's true identities...