Why titled Doppelganger ?
It took me a LONG time to find this movie on IMDB.
I remembered the title havig something to do with The Sun.
Now find it here as Doppelganger (1969)
aka: Journey To the Far Side of the Sun.
(Cool movie)
DOPPELGANGER was the original title, in Britain (it's a British film, of course), and as you know IMDb lists films by their original titles (even in foreign languages -- like "British").
JOURNEY TO... is the US title.
A "Doppelganger" is (I quote the dictionary) "A ghostly double or counterpart of a living person." Since you know the movie, the reference is obvious. Roy Thinnes even uses the word once in his dialogue.
You're right, very cool movie.
Also, his shuttle which crashes at the end of the movie is named "Doppleganger."
shareSpoilers!
Right -- a deliberate, ironic choice of his. Remember he tells the ground technician who asks why he had "Doppelganger" written backwards on the Dove (the shuttle), "It's a private joke between me and myself."
Personally I'd have had it written "backward" (i.e., normal for the second Earth) so that if he made it back to his own Earth they'd have proof that the other one exists. After all, he couldn't have repainted the name on the original ship himself.
Do you suppose that on that other Earth someone at this very moment is typing this message on (approximately) bDMI?