The far more common case is (distant) cousins marrying. Go back two hundred years, most working class people were born in one town, married in the same, and died there, so unsurprisingly you'll come across the same people in multiple branches.
Indeed, and there are also interesting modern cases where two gay men might get married, and adopt a child - in that case the child has no mother.
(Well obviously there is a mother in the biological sense, but it might be that the data isn't available. Instead in tracking a "family" tree you'd have two fathers.)
