IMO, it is extremely unlikely that you would not be detected by the time the flight is transferred to airport management. Long before the flight they tend to be a bit flexible because they know that people tend to change their travel plans and might make duplicate bookings, but as time gets near departure their computer system checks. it will not allow double booking under the same name/FFP number.
Getting the O award on a different flight could probably help. But keeping it as currently is calling for trouble. Most likely they will cancel one reservation and call for repricing on your cheap Y ticket. You might be very lucky and go under the radar (very unlikely) or be offered a nice solution at check-in, but that is not a risk that I would take.