This is a heavier question than you think. You'll need to do quite a bit:
- Set a cookie in the user's browser to keep track of the posts they've seen.
- Pull data out of that cookie to get a list of post IDs
- Add this to the array with the
$featured_post_id to pass it in the
I won't deal with the first two issues ... setting and reading cookies isn't a WordPress question and you can easily find that information with a simple Google search.
But for the rest of it, your code will look something like this:
// get_previously_viewed_posts() will retrieve a list of viewed post IDs from a user cookie.
$posts_to_exclude = get_previously_viewed_posts();
// Add the current post ID to the array of posts to ignore.
$posts_to_exclude = get_the_ID();
$args = array(
'cat' => '9',
'posts_per_page' => 5,
'post__not_in' => $posts_to_exclude
$my_query = new WP_Query( $args );
if ( $my_query->have_posts() ) : while ( $my-query->have_posts() ) : $my-query->the_post();
// the loop