Query for multiple tags, how to exclude duplicates

        <?php global $post;
        $nextTagThumb='-1';
        $yourCategory = '4';
        $tags = wp_get_post_tags($post->ID);
        foreach ($tags as $tag) :?>

        <?php
        if ($tags) {
        $what_tag = $tags[($nextTagThumb+'1')]->term_id;
        $args=array(
        'category__in' => array($yourCategory),
        'tag__in' => array($what_tag),
        'post__not_in' => array($post->ID, $do_not_duplicate),
        'showposts'=>20,
        'caller_get_posts'=>1
        );
        $my_query = new WP_Query($args); $do_not_duplicate = array();
        if( $my_query->have_posts() ) {
        while ($my_query->have_posts()) : $my_query->the_post(); if (!in_array($post->ID, $do_not_duplicate)) ?>

        <li class="post-<?php the_ID(); ?>">
        <a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><span class="sidebar-post-title"><?php the_title(); ?></span>
        <span class="sidebar-post-thumb"><img src="<?php echo get_post_meta($post->ID, "sidebar-thumb", true);?>" /></span>
        </a>
        </li>
        <?php endwhile;
        }
        wp_reset_query();
        $nextTagThumb = ($nextTagThumb+1);
        }
        ?>
        <?php endforeach; ?>