Originally Posted by
MaggieJune2
If he booked his new ticket on Delta though, would they realize he has two tickets booked out of the same city (for approximately the same time) and cancel one of them?
It is possible but unlikely unless the new ticket includes one segment being the same as the currently-booked ticket.
To reduce issues, don't include his SkyMiles number in the reservation until check in. That will assure they can't 100% identify the two reservations being for the same person (yes birthdates will be the same, but that is not a 100% assurance of the same person).