Offset posts with random order

Question

I'm trying to have five advertisement locations, each pulling posts from the same custom post type, but also loading randomly. The only problem I have with my code below is that it will sometimes duplicate the posts shown. Is there a way so that all 5 advertisements display a random advertisement post without duplicating?

Here's one of the banners:

<div class="advertisement">
<?php query_posts('post_type=advertisement_posts&offset=1&orderby=rand&showposts=1');?>

<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>
<?php $advert_url = get_post_meta($post->ID, 'rba_advert_url'); ?>
<?php $thumbnail_id = get_the_post_thumbnail($post->ID);
preg_match ('/src="(.*)" class/',$thumbnail_id,$link);
if(!empty($thumbnail_id)) {
$image_path =  thumbGen($link[1],234,116,"crop=1&halign=center&valign=center&return=1");
$image_all = get_bloginfo('url').$image_path;
$my_image = array_values(getimagesize($image_all));
list($width, $height, $type, $attr) = $my_image;
}
?>
    <?php if(has_post_thumbnail()) : ?>
    <a href="<?php echo $advert_url[0]; ?>" target="_blank">
    <img src="<?php echo $image_path; ?>" alt="<?php the_title(); ?>" width="<?php echo $width; ?>" height="<?php echo $height; ?>"/>
    </a>
    <?php endif; ?>

<?php endwhile; ?>
<?php endif;?>
<?php wp_reset_query(); ?>  
</div>

Any ideas?

Answer 1

Fetch all five posts in one query, store the result in a variable and take one each time you want to show an advertisement.

Now you cannot get duplicates, and more important: you save four queries.