On BA, I don't think there's any problem holding duplicate bookings, so long as they're booked directly with BA and the bookings have been ticketed. If you make a new Avios booking, that will be ticketed instantly, so both of your bookings will be ticketed and you should be OK so far as duplicate bookings are concerned. And that would give you leeway to deal with the booking that you don't want.

In any case, even on airlines that don't allow it, I think that these are usually picked up by robotics that do scheduled trawls. I don't think I've heard of an airline positively refusing to take a new booking merely because there is an existing booking on that flight. You can always try to circumvent that by booking without associating a name to a frequent flyer number. The system couldn't immediately know whether the "Mr John Smith" making a new booking now is the same person as the "Mr John Smith" who already has a booking on that flight - they could be two entirely different people.