add_filter(\'post_class\', function($classes){
  global $wp_query;
  static $meta = array();       

  // first call gathers meta values
  if(!$meta)
    foreach($wp_query->posts as $post)
      $meta[$post->ID] = get_post_meta($post->ID, \'your_meta_key\', true);

  // $wp_query->post should be the global (current) $post object
  $this_meta = $meta[$wp_query->post->ID];

  if($this_meta){
    $counts = array_count_values($meta);

    if($counts[$this_meta] > 1)
      $classes[] = \'your_class\';
  }   

  return $classes;
});