Recent Posts Help

I have set on my page to show the recentposts.

I have this:

<h3>New Articles</h3>
<ul  class="recent-posts">
<?php query_posts('showposts=10'); ?>
<?php while (have_posts()) : the_post(); ?>
<li><a href="<?php the_permalink() ?>"><?php the_title(); ?></a></li>
<?php endwhile; ?>
</ul class="recent-posts">

If i change (‘showposts=10’) with (‘showposts=20’) it really shows 20 posts ?? but some of them repeats 2-3 times.

How can i make it show last 20 or 30 posts without to repeat them 2-3 times?

Or “maybe” does they show 2-3 times because i have them posted 2-3 times same articles… (I’ll check and come back with an answer) ??

Thank You !