The Echo Maker by Richard Powers

Following a near-fatal accident, Mark Schluter is nursed by his reluctant sister. But when he emerges from his coma, Mark believes that this woman – who looks, acts, and sounds just like his sister – is really an identical impostor. As a famous neurologist investigates his condition, Mark tries to learn what really happened the night of his accident.

Shattered by her brother’s refusal to recognize her, Karin contacts the cognitive neurologist Gerald Weber, famous for his case histories describing the infinitely bizarre worlds of brain disorder. Weber recognizes Mark as a rare case of Capgras Syndrome, a doubling delusion, and eagerly investigates. What he discovers in Mark slowly undermines even his own sense of being.