Spock Must Die! (Star Trek)

Captain Kirk and the crew of the starship Enterprise find themselves in the middle of an undeclared war waged by the Klingon Empire...
The Organians should be consulted about the war but their entire planet has disappeared—or been destroyed...
Mr. Spock entered the transporter chamber. His image would be flashed to Organia by the huge machine's faster-than-light tachyons. But the experiment failed. Suddenly there were two Mr. Spocks. One of them had to be destroyed...
But which one?