Using multiple accounts as single person was a reason to get banned when I asked devs in pm years ago during 3rd bday event. I know people do it and arent banned to this day so yes its a problem.
You can improve ur own record as often as u want. Just the best one counts so it means once u have a better time the old, slower run will vanish from lb. You aren't limited in the amount of tries or the day you achieve the record.