I think you are thinking about it so much you are attracting it! When you aren't in a relationship you will see "couples" everywhere because you are focusing on what you don't have. Same with this scenario. I would yell out silently STOP whenever you are thinking about the other woman or him in a way you don't want. For instance them together. Break the cycle and replace it with a positive vision!
So I went into no contact mode with my target a few months ago because he told me about another woman and I felt disrespected by the entire situation and wanted to step back and decide whether I wanted to continue on or not. So recently, I've taken a break from pw/bwd and have just been focusing on myself and I have noticed that I have been seeing the others woman's name EVERYWHERE and it is a very uncommon name for a woman, I do think of her from time to time when I replay our conversations in my mind but I don't know anything about her or what she looks like other than her first name. Why is this happening? Update: today I saw the name of another girl he dated spray painted in the bathroom stall I was in, is the universe testing me to see if I'm really dedicated to changing my mindset?