One of favorite motifs is the evil twin replacing the original.

I think that the episode of Gilligan's Island with Ginger Grant and her evil twin Eva Grubb. Eva was an ugly duckling who wanted to escape from civilization, to be rescued they gave her a beauty makeover. She turned out to be a twin of Ginger Grant, both were played by Tina Louise.

Eva decided to replace Ginger and take over her life. So Eva hit Ginger with a coconut and tied her up.

Well Brittany Miller of the Chipettes has run into and afoul of her twin. She won't be singing tonight