This is going to be a bit long, and I'm posting it late, but anyway...

I met a couple former girlfriends through school and such, but not a lot happened until I started online dating. One of the first ones I met that way brought one of her friends along when we met in person because of course meeting a stranger from the internet was scary business (not that this has changed necessarily, but it was new at the time). Anyway after a date or two, things didn't work out with the first girl. I forgot her real name, but I'm making up names anyway so I'll call her Monica since mono=1.

But I later met the friend randomly at work! Small world. I'll call her Jenny. Her real first name is quite rare and would identify her rather precisely. So we started dating, and things seemed to be going ok, but slow. However after my dad died the relationship kinda fell off.

I went back to the dating site and eventually met another girl, I'll call her Amy. We scheduled a date for the local State Fair, and when the time arrived she was clearly really upset about something but still wanted to go on the date. So we did, and she was still visibly upset, but I didn't pry.
Later that day she eventually told the story:

Amy had a few months prior moved to my state from Georgia, and did so because of a boy she had dated before who moved here, but had friendzoned her. And that boy had started seeing someone, and Amy didn't like that, but it hadn't really hit her until that day, when the boy and the girl he was seeing had told each other that they loved each other. But Amy liked me and was trying to move on, so I did what I could to emotionally comfort her (we weren't physical).

Later on (forgot if same night) I invited her back to my place. Now of course I'm a geek and date geeky women, so when I say something stupid like "Want to see the cosplay pics I took at [the local anime convention]?" her response was "Sure".

As I was going through the pictures, she knew many of the characters and even already knew some of the people playing them. At one point as I was about to move on to the next picture her tone changed - she said "Oh, I know her". I raised an eyebrow and asked "Oh really?", and she said "Yes, that's the girl that [boy] is seeing".

After some effort to pick my jaw up from the ground, I managed to explain:
"She's my ex."
Amy asked, "Jenny‽" (Remember, unusual real name, which I hadn't told her)
"Yes." I said.

So while I was dating a girl that recently moved in, her ex was dating my ex. Going over the timelines later, Jenny was probably cheating on me before we broke up.

And no kids, Amy's not ~~your mother~~ my wife. This was many ~~seasons~~ years before.