<?php $do_not_duplicate[] = 0; $i = 0; ?>
	<?php $query = new WP_Query( 'posts_per_page=-1' ); ?>
	<?php if ( have_posts() ) : while ( have_posts() && $i<6 ) : $query->the_post();
		$category = get_the_category();
		if (!in_array($category[0]->cat_ID, $do_not_duplicate)) :
			$do_not_duplicate[] = $category[0]->cat_ID;
			$i++;
	?>
			<!--[HTML for show posts]-->

	<?php endif; ?>

	<?php endwhile; else: ?>
	<p><?php _e('Sorry, no posts matched your criteria.'); ?></p>
	<?php endif; ?>