I often book the same seats r/t. If you're going for the same long weekend, work week etc it's bound to happen.
I've seen people on my same flights back.
I sat next to a guy in the UC. We were seatmates on EWR-HKG. I've had the same purser in TG F (same route) and UA J...actually was also EWR-HKG and MUC-EWR.

I've seen people on my flights at a few of my hotels over the course of travels. Of all the tours in Saigon to take I had the same family on Cui Cui Tunnels and Mekong Delta Cruise (different tour companies).
I'm sure there are lots of coincidences we don't notice.