Mistaken Identity — Lisa Scottoline

Nothing can prepare criminal attorney Bennie Rosato for her new client, Alice Connolly, accused of murdering her lover, a highly decorated police detective. Connolly, who bears an uncanny physical resemblance to Bennie, tells the astonished lawyer, "Pleased to meet you. I'm your twin. "But Bennie grew up an only child. She doesn't have a twin.
Or does she?

Bennie takes the case and plunges into the mystery of the murder, as well as the secret of her own identity. Not until the verdict is in will she finally learn the truth.