Doppelgänger, aka Journey to the Far Side of the Sun (1969), a British film directed by Robert Parrish, produced by Gerry and Sylvia Anderson. With Roy Thinnes (the American star of The Invaders series).
A planet is discovered in the same orbit as Earth’s but is located on the exact opposite side of the sun, making it not visible from Earth. The European Space Exploration Council decide to send American astronaut Glenn Ross and British scientist John Kane via spaceship to explore the other planet.
This film made a lasting impression on me when I was young. The effects are good for the time and the story is original. I didn’t see it again. I don’t know if it aged well, but it’s really unique.