James Blish

"That's all there is to it," Scott said. "He can come out now.'
Spock, however, failed to appear. Kirk said, "We seem to forgotten to arrange any way to let him know that. I assume it's safe to open the door now?"
"Entirely, Captain."
Kirk went to the platform and slid the door back. "Mr.Spock..."
Then he stopped. Spock was there, all right, and apparently quite unharmed. In fact, he was one hundred per cent too much there.
There were two identicals Spocks in the tank.