some psuedo code to get you going on posts your users have already seen

pseudo_code.php

<?php // the cookie should hold an array of post_ids ?>
<?php $seen_posts = unserialize( strip_tags( $_COOKIE["seen_posts"] ) ) ?>
 
<?php 
// set up your WP "loop"
foreach $loop( "SELECT * FROM POSTS WHERE post_id NOT IN ( $seen_posts ) "): ?>
<?php
	// you loop is empty. The SQL query returned no records
	if( empty($loop) || count($loop) == 0 ){
	   $seen_posts = array();
	   // NOW start the loop again
	}
?>
	... looping code
	
	<?php array_push( the_id, $seen_posts ) ?>
<?php endforeach; ?>

<?php setcookie( "seen_posts", serialize($seen_posts) )?>