Alternate versions for
Journey to the Far Side of the Sun (1969) More at IMDbPro »Doppelgänger (original title)

US prints give Roy Thinnes top billing over Ian Hendry, perhaps due to Thinnes being known for his TV series "Invaders, The."
Screenings of Doppelganger on British terrestrial television over the last ten years have featured an incorrectly produced print: the person responsible for creating this print was unaware of the film's central premise and performed a second "flop-over" on the 'mirror' planet footage, in the mistaken belief that it was the wrong way round. This, of course, made nonsense of this section of the film (particularly Ross' slow realisation that he has not returned to Earth after all) and thereby gave viewers the impression that they were watching the 'mirror' planet's Ross here on Earth.
Some British prints also featured an additional voice-over during the climax: in the corridor at the nursing home, as he reaches out for the mirror Webb hears Ross' voice telling him, "Jason, we were right. There are definitely two identical planets." (Taken from earlier in the film, this is a line of dialogue that Webb never heard spoken as communications between Webb and Ross were cut mid-sentence.)