I don't know that it is so much falling for someone who looks like someone we may have once had a crush on, as it would be falling for someone who fits the type of person we are attracted to. I know that most of us have a mental image of the person we see ourselves with, and subconsciously look for that person in every face we see, if we don't already have them in our lives.
Though, in breaking of type, if you fall for someone who is not the norm for what you are attracted to, and then see that person's face in others who have similarities to him/her, then you are probably reliving some of the memories of your original crush through the familiarity of seeing someone who looks so similar to your past love. If you were to get together with this new person, would you be able to separate the two as two distinct people? Or, would you project habits and behaviors of the former onto the new partner, along with any expectations held and not lived up to by your first love?