Oh brother, where art thou? Well, if he has an identical twin, and it’s early August, he’s probably with him at “Twins Days,” the largest gathering of twins anywhere in the world. In his extraordinary short subject documentary “Seeing Double,” filmmaker Mark Wexler visits the annual festival, held in Twinsburg, Ohio every August, and takes us on a memorable journey into the odd, and oddly endearing, culture of twinhood. Interviewing a number of siblings attending the festival (all of whom not only look alike but dress alike), Wexler reveals the curious, even eerie bond that links twins throughout their lives, from the private invented languages of childhood, through the strange phenomenon of shared physical pain, to the troubled marriages that sometimes arise from intense spousal jealousy. Aired on the National Geographic Channel, “Seeing Double” is a celebration of life, of brotherhood and sisterhood, and of the mysterious connections that make the human family an everlasting riddle.