Let's say that you've got this ex-boyfriend, or ex-girlfriend - and you have fond memories of one another. But, unfortunately you didn't end up together. The odds were stacked against you anyway - maybe you were too young, maybe you wanted different things out of life - maybe one time you thought about trying again, but the timing wasn't right. You were in another relationship, or maybe he/she was. Whatever the situation, it just wasn't in the cards for you as a couple.
So you both go on in your lives, and you both find a partner. Someone you choose to settle down and start a family with. One of you has children first...then the other follows suit. Because you both have mutual friends and acquaintences, while you don't speak to each other, you're both aware of what each of you is doing in life.
Say one of you has a daughter, and names her "Sarah Anne". Eighteen months later, the other person has a daughter - and names her "Sara Ann".
Mere coincidence.... or wistful thinking of a life one wishes they had?