I've done it on UA (sorry, not a Delta data point) more times than I want to admit. It stems from trying to book a few flights and invariably gloss over that I accidentally chose a premium seat (for an outrageous price that I'd get for free when checking in, since I'm only Silver there). UA is adamant that they can't refund the upgraded seat price even though the flight could be many many weeks out, but I digress.

They've allowed me to book multiple flights for myself MANY time w/ the exact same itinerary. The one time it was "caught" was weeks later a factor of when I cancelled myself via the link, it "must not have gone thru" Other times I've usually waited 2-4 days before cancelling to see if they auto. cancelled one for me...never have. All the above flights were booked either pre-Aug of this year and spread throughout last year. I did have one a week ago, almost, but closed the browser window before all info could be sent (apparently)...need to double check my CC stmt.

Good luck