Opposite sex doppleganger, this is difficult to come up with a definitive answer. There's some dudes on here that have some things in common with me, but aren't INFJ, like @phobik @Qlip that are synth nerds. But I don't see them as being exact duplicates of me, both being extroverts.
Typologically speaking, the closest is @Peguy, being both INFJ E-4's with a scholarly bent. But he's a history/metaphysics buff; whereas, my areas of expertise are in science and music. There's overlap/parallel in ideas, but we examine things from different angles.
I find that I have more same-sex dopplegangers here.