Can you register a marriage twice (in UK and US, Nevada) to the same person?

The U.S. does not have a national "registration" system.  Entering into a Nevada wedding should not have any effect on your visas.  It is a bit unclear what you are trying to do and why.  If you are lawfully married in the UK, you are considered married here.  If not, not.  You can have a ceremonial marriage here, and celebrate it any way you wish -- just call the chapel/venue where you wish to have your Vegas marriage and explain the legal situation, and they should be able to accommodate you.  People do "commitment ceremonies" and other "weddings" that do not formally have a legal effect all the time -- the fact that you are already legally married should not be an impediment to the ceremony you plan.