create classes script include . 

sample steps :

url :

sys_user_list.do?sysparm_query=user_name=javascript:getDupUsers()


classless script include. 

function getDupUsers() {
  var usernames = [];
  var grUser1 = new GlideRecord('sys_user');
  grUser1.addActiveQuery();
  grUser1.addNotNullQuery('user_name');
  grUser1.query();
  while (grUser1.next()) {
    var username = grUser1.user_name.toString();
    var grUser2 = new GlideRecord('sys_user');
    grUser2.addActiveQuery();
    grUser2.addNotNullQuery('user_name');
    grUser2.addQuery('user_name', '=', grUser1.user_name);
    grUser2.query();
    var len = grUser2.getRowCount();
    if (len > 1) {
      usernames.push(username);
    }
  }
  return usernames;
}

Note: same way you can design it for your requirement. hope sample code will help you.