Duplicate posts appearing in category.php after upgrade to 2.6.5

We just upgraded from wordpress 2.23 to 2.6.5.

The problem appears here: https://briefingroom.thehill.com/category/news/page/3/

If the post has more than one category, often a sub-category of “NEWS” then the post will duplicate. Workaround is currently to only assign one category. However, issue did not appear until after the upgrade.

I have tried switching themes, the problem still appears under default/classic themes.

here is the loop inside the category.php file.

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<?php the_date('','<h2 id="date">','</h2>'); ?>
<div>

  <h3 class="storytitle" id="post-<?php the_ID(); ?>"><a href="<?php the_permalink() ?>" rel="bookmark">
    <span class="bluetitlecenter"><?php the_title(); ?></span>
    </a> </h3>
  <div class="meta"> @
    <?php the_time() ?>
    by
    <?php the_author_posts_link('namefl'); ?> <?php edit_post_link(__('Edit This')); ?>
  </div>

                <span class="greycopy"><?php the_content(__('> Read More')); ?></span>
        <div class="feedback2">
            <?php _e("Archived under:"); ?>
            <?php the_category(',') ?><br />
                        <?php the_tags();????>
            <br />
    <?php wp_link_pages(); ?>
    <?php comments_popup_link(__('Post a Comment (0)'), __('Post a Comment (1)'), __('Post a Comment (%)')); ?>
    <br /><br />
    <img src="/wp-content/themes/briefingroom/images/double_lines.gif" width="400" height="4" /><br />
    <br />
  </div>
        <!--
        <?php trackback_rdf(); ?>
        -->
</div>
<?php comments_template(); // Get wp-comments.php template ?>
<?php endwhile; else: ?>
<p><?php _e('Sorry, no posts matched your criteria.'); ?></p>
<?php endif; ?>

any assistance is most welcome.

thank in advance!!!!