The Doppelgänger 

Commander John Koenig hesitatingly groped forwardly 
along. All his senses were strained. The glasslike 
walls of the cave path radiated an unreal light. 
As Koenig heard the steps and reacted, it was already 
too late. 
The man, who faced him, was his exact image. Koenig 
had the feeling of looking into a mirror. But his alleged 
mirror image led a weird independent existence. The face 
was distorted by hate. Before the genuine Koenig could 
draw his lasergun, his image fell on him. 
The hands of the Weird One were like steel clamps around 
the neck of the Commander...