The goblin reservation (1968)

Having just returned to Earth from an intergalactic research mission, Professor Peter Maxwell, specialist in Supernatural Phenomena, finds himself in most desperate straits. Earth, as the professor is well aware, has advanced beyond belief in many areas; time travel, for instance, has been perfected to such an extent that creatures from different ages (goblins, dinosaurs, and Shakespeare!) live together at the same time. But Maxwell has accidentally discovered a mysterious crystal planet which contains a storehouse of secret information, and he is sure that not even one-tenth of this knowledge is yet known on Earth. Realizing the value of the planet for the Earth's future development, he attempts to convince those in power that they must, at any cost, get control of the crystal planet. 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 new Maxwell really exists.