downFast on "Show only unique tags"

The following shows me all tags names based on all posts, which means I end up with a list with duplications. How do I stop it so that it only shows me unique tags?

<ul>
            <?php
                query_posts('category_name=html');
                if (have_posts()) : while (have_posts()) : the_post();

                    if( get_the_tag_list() ){
                        echo $posttags = get_the_tag_list('<li>','</li><li>','</li>');
                    }
                endwhile; endif;
                wp_reset_query();
            ?>
   </ul>

Any help please?