Infinity Beach by Jack McDevitt

Originally published: New York : HarperPrism, 2000.

After centuries of searching for extraterrestrial life, it is determined that we are alone in the universe. Or so it's believed, until Dr. Kimberly Brandywine sets out to find out what happened to her clone-sister, who disappeared along with the rest of her ship's crew after the final, unsuccessful manned SETI mission. The truth may be more terrifying than Kim ever dared imagined.