Avoid Duplicate Posts in Multiple Loops

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<h2>Loop n°1</h2>
<?php
$ids = array();
while (have_posts()) : the_post();
the_title();
?>
<?php $ids[]= $post->ID;
endwhile; ?>

<h2>Loop n°2</h2>
<?php
query_posts("showposts=50");
while (have_posts()) : the_post();
if (!in_array($post->ID, $ids)) {
  the_title();?>
<?php }
endwhile; ?>

Instructions:

Let’s start by creating a simple PHP array, and put all post IDs from the first loop in it.

Now, the second loop: we use the PHP function in_array() to check if a post ID is contained in the $ids array. If the ID isn’t contained in the array, we can display the post because it wasn’t displayed in the first loop.