Select all the brushes and actors you want to duplicate (if the one level is all that your map consists of, then the 'select all' feature would help), then use the duplicate command (ctrl-w). Move the newly generated copy of everything to where you want it to go (check your alignment!) and de-select.
Also, you should try to vary the decor in additional levels, or use labels or some other method so the player knows what level they are on. With multiple levels that are perfectly identical, it could become confusing in a hurry.