American Hominid said: The only other character who has a double/fake name in the OT that I can think of is Ben Kenobi, and Ben doesn't seem to be a title for him. So 'Darth' needn't, from the OT alone, be a title (though it works well as one).
Furthermore, Obi-Wan says, "A young Jedi named Darth Vader." The implication is that "Darth" is a first name, not a title.