aka The Night of the Puudly / The Duplicate Man
Story, first published: Galaxy Science Fiction, March 1951
The chilling account of Henderson James, an alien psychologist suffering from temporary amnesia. When his memory returns, he remembers he must seek out and kill a "puudly", a dangerous alien whose instincts for self-preservation make it hate all living things. A "puudly" is difficult to kill because it can implant thoughts into its victim's minds. The situation is even more urgent because the "puudly" is about to bud. James is able to kill the alien, but with its dying thought, the puudly calls James a "half-thing, a duplicate". James realizes that if he goes home, he will be killed because "duplicates" are not allowed to live after they have served their purpose. James has two choices if he wants to remain alive - either confront the original Mr. James and make a deal or flee. He chooses to see James, sneaks into his house, but finds it empty. The duplicate James orders the execution of the original (and here it is almost a shame to give away this O. Henry ending in this plot summary). Suddenly the telephone rings - it is the duplication lab. They did this duplicate differently - "an experiment, slow poison in the bloodstream ... twenty-four hours ... like a time bomb."
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.57-58