A student will appear twice if the student submitted two registrations.  The most likely cause is that the student noticed an error and went back to fix it.  One of the records needs to be deleted.  The best bet is to delete the first record in case they made a change and then submitted the second.  You may do this at "My Report Center" by clicking on "Database Management".  You can use the time stamp to see which is more recent (keep the most recent).