I had this happen and had to call reservations to book the new ticket. There wasn’t anything else I could do on my end. I had cancelled it and removed it from my trip list but this didn’t free up the dates. This is annoying as fares were changing and reservation agents won’t see your fare until you get the the stage where you can put the itinerary on hold. Of course, you can’t get to this step as you get the duplicate booking error. So you have to hope the fare is still available by the time you get to talk to someone.