Well, I happen to be a self-appointed expert on the subject of Doppelgänger (one's "double" in life, or the "lookalike" phenomenon), as I've dealt with the issue somewhat constantly since I was about 20!
Seriously, to what most people casually react when they say "you look like..." is some single aspect of one's face which does indeed resemble the similar aspect of the other person's face, which prompts the "lookalike" remark, and for some reason (facial recognition circuits in the brain, which are indeed complex and complicated) makes them think you are a "double," when usually one in objective fact is not. For example, one's mouth (lips) and smile might be similar enough to prompt the brain's circuity to signal "recognition." (DestinHarbor's comment about being mistaken for Robert Redford is interesting: I would never have thought that initially, but having been prompted by the possible resemblance, I can also see the aspect which prompts the resemblance comments. However, DH is quite a handsome man in his own right.)
For people who get continually told that "they look like...," the commenters are usually reacting casually without deep analysis to one particular facial feature, especially if it is a prominent feature, which overrides careful consideration objectively of all aspects of similarity. Personally, that particular prominent feature of mine has been my very prematurely white hair; consequently, I've been told I "look like" a whole series of celebrities, major and minor, based on this single white hair feature. I almost got into a bar fight once because this tipsy dude was insisting I was this at-the-time-quite-famous entertainer and was rudely denying tipsy dude the opportunity of meeting someone famous. Eeech! Particular annoying, especially since I neither thought I looked like the celebrity nor did I particularly like him (although he is talented). Other proffered "lookalikes" have had absolutely NO resemblance except for the white hair bit (among others, think very prominent gay newscaster).
Looking at pics of guys on social media, I very occasionally see pics of guys who initially my facial recognition circuitry remind me of my younger self, but on closer examination I would not say they were "lookalikes."
However, most probably, somewhere on earth there does exist one's true Doppelgänger. I do have mine—an European dude of approximately the same age with whom I most probably share little to no close genetic background, who looks enough like me that I myself have mistaken his picture momentarily for being a picture of me, when I haven't seen the picture recently enough to have it in mind. I first learned of the picture when my friends, who otherwise never would kid me on such a subject, started teasing me about being on the cover of an internationally famous magazine. Sure enough, when I finally saw the cover, I was shocked! The guy was about as close a facial double as one could get, and certainly close enough to fool even my own facial recognition circuitry on occasion. We were just a wee bit short of being identical twins facially.