Spock Must Die! by James Blish

In this gripping science fiction tale, the crew of the USS Enterprise faces a moral and existential dilemma when a transporter accident creates a duplicate of their beloved Vulcan science officer, Spock. As the crew grapples with the implications of having two Spocks aboard, tensions rise and loyalties are tested. With the fate of the universe hanging in the balance, Captain Kirk and his team must determine which Spock is the original and which is the imposter, all while navigating the complexities of identity, ethics, and the very nature of existence itself.