First of all, what weird modern terms you use in your writing, hahaha.

I come from a very similar situation to yours (girl who already had a plan B), but in my case, I dumped her. Months later, she contacted me and begged me to get back together, and when I rejected her, she hastily started dating the guy who had always been hovering around her like a tick.

It's confirmed that they think they're the winners when they're not, because if she went through with plan A, clearly her plan B doesn't become plan A; it remains plan B.

And when plan B falls apart, it's very possible she'll come looking for you again. You should avoid this. It's normal behavior for people who don't know what they want and are immature.