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The superstar acting and screenwriting duo are actually related, research on their respective family trees revealed this week.
Damon and Affleck are both descendents of William Knowlton, Jr., a bricklayer who settled in America after moving from England in the 1630s, New England Genealogical Society researchers said Friday.
The pair are tenth cousins once removed, according to the researchers.
Affleck also has a bevy of other A-list relatives, including the late Princess Diana and 16 U.S. presidents -- including Barack Obama.
Affleck and Damon are childhood friends who met while growing up in Boston and gained fame together as Oscar-winning writers of the 1997 hit "Good Will Hunting," which starred Damon as a janitor with a genius I.Q.