related posts by tag

<section class="similar">
    <h4>Similar</h4>
    <ul class="posts">
    <?php
        $tags = wp_get_post_tags($post->ID);
        if ($tags) {
            foreach( $tags as $tag) { $first_tag[] = $tag->term_id; } //NEW
            $s_args=array(
                'category_name'         => parent_category(),
                'tag__in'               => $first_tag, //NEW CHANGE
                'post__not_in'          => array($post->ID),
                'showposts'             => 4,
                'caller_get_posts'      => 1
            );
        $similar = new WP_Query($s_args);
            $not_to_dupe = array($post->ID); //NEW
        if ($similar->have_posts()) {
            while ($similar->have_posts()) : $similar->the_post();
            $not_to_dupe[] = $post->ID; //NEW
                get_template_part('loop', '');
            endwhile;
        };
    }; ?>
    </ul>
</section>

<section class="popular">
    <h4>Popular</h4>
    <ul class="posts">
        <?php
            $p_args = array(
                'category_name'         => parent_category(),
                'orderby'               => 'comment_count',
                'post__not_in'          => $not_to_dupe, //NEW
                'posts_per_page'        => 4
            );
            $popular = new WP_Query($p_args);
            while ($popular->have_posts()) : $popular->the_post();
                get_template_part('loop', '');
            endwhile;
        ?>
    </ul>
</section>