it's inherently possible to have two reservations on the same flight under the same name. think common names like john smith.

the flags at check in will be if the birthdate and mileage plus numbers are the same on both reservations. i predict that at checkin you will have some problems given that both will be true. i'd just proactively cancel one of them now, you can try for the refund by phone -- you likely won't get it. if no refund, canceling by phone or online on UA itinerary page has no difference in consequence.