Related Post double value

I am working on this code to show my related by any tag posts, but its showing me thumbnail as a link twice for posts with the same tag.

How can i force this to show related post only once?
If you have >=2 post with 2 the same tags you will get doubled values

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

<?php
    if ($tags) {
        $what_tag = $tags[($nextTagThumb+'1')]->term_id;

        $args=array(
            'tag__in' => array($what_tag),
            'post__not_in' => array($post->ID),
            'showposts'=>100,
            'ignore_sticky_posts'=>1
        );

        $my_query = new WP_Query($args);

        if( $my_query->have_posts() ) {
            while ($my_query->have_posts()) : $my_query->the_post(); 
?>
            <li>
                <a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>">
                    <?php the_post_thumbnail(); ?>
                </a>
            </li>

<?php 
            endwhile;
        }

        wp_reset_query();
        $nextTagThumb = ($nextTagThumb+1);

    }
?>

<?php endforeach; ?>