Prevent Duplicate Post Counted by Query

<?php  while ( have_posts() ) : the_post();           $  get_series_id = get_post_meta( get_the_ID(), 'series_seri', true );          if (in_array($  get_series_id, $  do_not_duplicate)) {             continue; // We've already seen this post ID, so skip the rest of the loop         }      $  do_not_duplicate[] = $  get_series_id; ?> <?php the_title(); ?> <?php endwhile; wp_reset_postdata(); ?> 

how to make duplicate post not counted ? i wanna show 20 post, but in frontpage only showing 5 post because duplicate post also counted,