In romantic triangles, it would certainly make sense that the main girl should be most threatened by a side-chick who resembles her. If the guy has certain turnons, conscious or sub-conscoius, then a similar-looking side-chick will have those same properties.
I especially enjoy the scenario where a side-chick CHANGES her appearance (hair color, hair length, breast size) to match her rival. It's a way of calling out the main girl.