https://www.newyorker.com/magazine/1995/08/07/double-mystery
Recent studies of twinship have challenged our most entrenched views of human development and have capsized cherished beliefs about human nature—in particular, the bedrock notion that character is created by experience. But then twins have been confounding humanity from the earliest times—almost as if they were a divine prank designed to undermine our sense of individuality and specialness in the world. Twins are both an unsettling presence, because they sabotage our sense of personal uniqueness, and a score-settling presence, because their mere existence allows us to pose questions we might not have thought to ask if we lived in a world without them.