For more than a decade, Boris Furman has meticulously tracked the whereabouts of his family members, averaging the latitude and longitudes to arrive at “The Family Average Location.” But nobody really knows why.

Being an identical twin is kind of like having a parallel world right on top of ours, one in which there is another version of you running around. Dana Chivvis has the story of the Sklar twins, and a 48-year-old mystery.