Similitude (Star Trek: Enterprise, Season 3, Episode 10)
Commander Charles "Trip" Tucker is left in a coma after an explosion while he's working on the warp drive. Doctor Phlox suggests that the only way to save him is to clone him with a "mimetic symbiote." Captain Jonathan Archer is hesitant but ultimately goes forward with the plan when he accepts that the ship is in danger. The clone will only live for 15 days, and Phlox can harvest neural tissue from it to bring Tucker back to life. However, the clone -- named Sim -- learns what he is and what his fate will be, and he and the entire crew have to decide how to proceed.
Watch Star Trek: Enterprise on CBS All-Access.