First rule: Never mix old wine with new wine (an old saying)! I believe a person who doesn't possess all the required components of "true love" for his/her partner, will be the one who is likely to find him/herself in the above predicament. When you are lacking even one component of love(commitment, Passion and Intimacy), you will always find yourself "flip-flopping" in relationships.
What I am thinking here, is that, anyone who is in this situation, is still thinking about the past person, while holding on to the present person simply because they both possess some of the qualities or characteristics that are important to him/her. Obviously, this new person is not satisfying all the desires or needs! This profess "love" for this new person is not a real love( not a commensurate love), this is why doubts are still lingering.