Back in the day, I used to double book flights on occasion. Mostly for business... when I had a flight booked that I knew I could make and one that I hoped I could make. Once the new system came on line, those double booked were auto cancelled.

I did have One situation recently that I needed to double book flights. It was a situation where I either had to go to “place a” or “place b” on a certain day. I booked both and called Southwest. They “flagged” “the system to prevent the auto cancel. I ended up cancelling one of them a few days before the flight.