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when two people realize they've kissed and/or made out with the same person.

addendum: the closeness of the relationship is dependent on the length of time passed between said incidences. Like, if two people "crossed paths" over the course of a week, it would be known as "lip twins." But if it occurred years apart it would be "lip cousins."
"ashley! i can't believe we made out with the same boy!"
"yeah. we're totally lip cousins."
by tengelke May 12, 2012
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