Clifford D. Simak 
Goblin Reservation 

Having just returned to Earth from an intergalactic research mission, Professor Peter Maxwell, specialist in Supernatural Phenomena, finds himself in desperate straits. Earth, as he is aware, is well advanced in many areas; perfected time travel, for instance, enables all creatures (goblins, dinosaurs, and Shakespeare!) to coexist. But Maxwell has accidentally discovered a mysterious crystal planet containing a storehouse of secret information not yet known to Earth. Knowing the value of the planet for the future of Earth, he attempts to convince those in power that they must, at any cost, get control of it. But his efforts are thwarted by a startling fact: Maxwell was ingeniously duplicated on his return trip. The ‘other’ him came back before he did, and was soon after ‘accidentally’ killed. Now no one will believe the original Maxwell exists.