Sort users by unique sities

Hello,

i have to sort the users by city. but i have to show only unique cities (no duplication)

i created this code which sort and display me list of the cities, but i have to get rid of the duplicated

<?php

$args  = array(
  'fields' => 'all',
  'role' => 'Subscriber',
  'meta_query' => array(
  					array(
						'key'=>'city'
						))
  );

$blogusers = get_users($args);

//custom function for comparing the data we want to sort by
function cmp($a, $b){
  if ($a->city == $b->city) {
    return 0;
  }
  return ($a->city > $b->city) ? 1 : -1;

} 

usort($blogusers, 'cmp'); 

foreach ($blogusers as $user ) { // get all the user's data

$city = $user->city;

print '<h2>' . $city . '</h2>';

}

?>

the result:

Atlanta
Atlanta
Atlanta
Boston
Chicago
Detroit
New York
San Jose
Washington DC
Washington DC
————–

what i need is:

Atlanta
Boston
Chicago
Detroit
New York
San Jose
Washington DC

———–

please help

thanks