Double Helix, Book 4.
Like intertwining filaments of human and alien DNA, a ruthless campaign of revenge has threaded its way through the galaxy, touching billions of sentient beings – and changing forever the life of Lieutenant Thomas Riker.
Tom Riker, an identical duplicate of the Starship Enterprise's first officer, is serving as a Starfleet medical courier when he encounters a group of Maquis renegades, led by a former Starfleet officer named Chakotay. A planet in the Demilitarized Zone, now controlled by the Cardassians, has been stricken with the same deadly disease that has plagued the Alpha Quadrant for years, and only Riker can get the medical supplies the Maquis so desperately need. But the Cardassians would rather destroy all life on the planet than risk letting the epidemic spread!