Have you ever seen someone you thought you knew, maybe from a short distance, and you thought for certain that you were seeing a friend? The hair, the height, the way they walked, and even the clothes they wore... all these things seemed to match the person you thought you were seeing... until that is, you get close enough to realize that it was actually somebody else who merely looked like the friend or acquaintance y0u thought you'd seen.
This kind of phenomenon has lent itself to a variety of interpretations of what might be called doppleganger phenomenon, as well as philosophical notions of what exactly consciousness is, and how we use it to relate to others around us. Often, it seems that there is something fundamentally deeper about the nature of the human experience, and that rather than just being physical bodies moving around and interacting with one another on a day-to-day basis, there indeed might be more to the proverbial pie than just the aroma we're able to catch from time to time... especially when it comes to strange phenomena.
For instance, from a rather personal perspective, I've often likened various past loves that have come and gone to being repeated manifestations of a single sort of greater feminine archetype I've encountered, rather than merely being individuals I've known over the years. Sometimes, I've even encountered strange sorts of synchronicity and other manifestations of a curious nature in this regard: one girl I had known, for instance, took to calling me by a nickname which a previous girlfriend had used for me, with no prior knowledge of that nickname being appended to me in the past. Granted, I'm not literally suggesting that every girl I've dated over the years has been the same woman in some surreal, cosmic sense. However, I think that in terms of Jungian psychology, there are from time to time various "manifestations" of things that are familiar to us, shades of which might occasionally pour through the fabric of physical existence between people we know, revealing themselves in startling ways.
I bring up Carl Jung here since it was he who first supposed that all people might be interconnected by a greater "collective subconsciousness", as he called it. This sort of thing, viewed from the perspective of being a phenomenological manifestation of our inner selves that all people share, is a bold and difficult thing to accept scientifically; nonetheless, Jung used this hypothesis for the justification of random coincidences that seemed to have much deeper symbolic meaning (often related to him by his patients), in addition to a possible way to explain various psychic phenomenon, or even more complex notions that are typically relegated to the realms of religious spirituality, such as past lives and reincarnation.
Indeed, I do often feel that I catch "shades" of people I know or have known in others I meet. In terms of evolution, this literally could come from the limitations of diversity that exist, in actuality, within our own genetic makeup. In other words, physical notions of individuality may exist only within the given confines of a number of random physical traits which do tend to pop up continually from person to person. Hence, we're brought back to the notion of "dopplegangers" from earlier; if indeed there are a variety of traits--though perhaps somewhat limited in scope and number--which represent all the potential genetic differences that can occur in a human being, it stands to reason based on probability that those sequences might randomly produce, on occasion, two individuals that bear remarkable similar traits (hence, we think we see someone we know, when in reality, they merely looked a whole lot like that person we thought we were seeing).
But taking this a step further, what if these same sorts of limitations on variety could occur in other ways, perhaps even those that are non-physical? What if there could be predetermined elements about human existence that, in addition to governing how we physically look, might also dictate certain aspects of who we are spiritually, how we perceive the world around us, and in some capacity that is yet-to-be-understood, perhaps even tie us one another subconsciously? Given this criteria, perhaps there is far more to the notion of dopplegangers, or even the sense of "knowing" aspects of a person as if by psychic means. We could all just be running in peculiar little spiritual circles, meeting one another again and again in a bizarre, cosmic interplay that has been occurring since time immemorial... and which will continue on in the same fashion.