The third is the discovery by space explorers of another world where we meet our doubles: this is what happens in a mysterious fashion in Kubrick's 2001: A Space Odyssey and Tarkovsky's Solaris and more explicitly in Journey to the Far Side of the Sun (aka Doppelgänger), the 1969 venture into live-action film‑making by the Thunderbirds team.
Rhoda and the audience learn about this "mirror Earth" via the patter of a hip-hop deejay on a car radio; this is a beautiful example of the film's recurring success in conveying important information in believably mundane ways, without the need for science-fiction special effects or sequences with scientists or feds, as one might find in a studio film: I'm thinking in particlar of 1969's "Journey to the Far Side of the Sun," a more traditional if admirably "Twilight Zone"-esque science-fiction film about the discovery of a duplicate Earth.