navigation broke in posts in two columns

After several trys, I got a index of 2 posts columns. It seemed to be everything ok, but it didn’t: the navigation broke, I click and click but I always get de first page, I can not go back to de older posts. I think it would be because of the way I code the 2 columns loop, but I am not sure and I do not know how to fix it. Does anybody help me?

This is a extract of my index’s code:

<div id="content">

<div id="col1">
	<?php if (have_posts()) : ?>

	<?php $my_query = new WP_Query('category_name=Sociedad&showposts=8');
  while ($my_query->have_posts()) : $my_query->the_post();
  $do_not_duplicate = $post->ID;?>

	<?php endwhile; ?>
			</div>

<div id="col2">

	<?php query_posts('cat=-2&showposts=9'); // exclude the first category ?>

	<?php while (have_posts()) : the_post(); ?>
			</div>
	<?php endwhile; ?>

	<div class="navigation">
			<div class="alignleft"><?php next_posts_link('&laquo; ART&Iacute;CULOS ANTERIORES') ?></div>
			<div class="alignright"><?php previous_posts_link('ART&iacute;CULOS POSTERIORES &raquo;') ?></div>
	</div>
</div>

	<?php else : ?>

		<h2 class="pagetitle"><?php _e('Resultados de la b&uacute;squeda'); ?></h2>
		<?php _e('No aparecen resultados con las palabras utilizadas'); ?>

		<?php include (TEMPLATEPATH . '/searchform.php'); ?>

	<?php endif; ?>

</div>