The Barbie Murders by John Varley

A detective from outside a secluded, near-identical community must unravel the mystery of a brutal killing among a group of people who deliberately present themselves as indistinguishable—physically and socially—forcing her to confront cultural norms about identity, culpability, and justice as she struggles to find a trail in a society that erases individuality and has its own rules for dealing with transgression.