we added a custom query to our template which works fine. So we want to display all posts out of our array. But if we have some duplicate IDs in our array, WP ignores the duplicated IDs and shows every Posts->ID only once.
$args = array( 'post_type' => "custom_posttyp", 'order_by' => 'post__in', 'post__in' => $post_ids, // array with (208, 212, 218, 208, 212, 218) );
So based on this example the while-func just renders 3 posts instead of 6 – we think because of duplicated IDs – but we sometimes have to render also 2 or 3 copies of one post if the array has IDs twice, three times etc.