Fairview said:
Can't speak for all airlines but United will send you an email within an hour or two of buying the second ticket and say they will cancel one if you don't.
Maybe if you called and said you were buying it to have extra space they'd be ok with it.
bthotugigem05 said:
It's possible but needs to be handled by an agent. Most airlines automatically cancel duplicate flights booked for the same date.
United literally has their system set up for someone to do this easily without calling. On your second/third, etc traveler, next to the name pull down, is a box for 'this is an extra seat'