Dr. Maura Isles, the chilly medical examiner introduced in last year's ''The Sinner,'' is back! And she's got a hot twin! Who's...dead. Isles returns home from a business trip to discover the body of the doppelgänger she never knew she had. Turns out the siblings may be the offspring of a convicted murderess. The story zips along, introducing the abduction of a pregnant woman and a delightfully bizarro plot twist. But the face-offs between Isles and her killer mom -— which should have smacked of ''The Silence of the Lambs'' -- fall flat. Tess Gerritsen's Body Double works the brain a bit, but unfortunately leaves the nerves intact.