Spock Must Die! by James Blish -- 128 pages
When a transporter experiment goes horribly awry, suddenly there are two Mr. Spocks! One is the true First Officer of the Enterprise. The other is his complete opposite, a traitor whose very existence poses a grave threat to the crew, the ship, and the Federation itself. One of the Spocks must die. But which one ... ?
This is book 1 of the Star Trek Adventures.