The Likeness by Tana French

Still traumatised by her brush with a psychopath, Detective Cassie Maddox transfers out of the Murder squad and starts a relationship with fellow detective Sam O’Neill. When he calls her to the scene of his new case, she is shocked to find that the murdered girl is her double. What’s more, her ID shows she is Lexie Madison – the identity Cassie used, years ago, as an undercover detective. With no leads, no suspects and no clues to Lexie’s real identity, Cassie’s old boss spots the opportunity of a lifetime: send Cassie undercover in her place, to tempt the killer out of hiding to finish the job.