Do not duplicate posts

Hey guys, i have a problem with this issue:

My homepage are looping duplicate posts. The same post shows 3-4 times, and at total, i have around 40 posts.

I’ve tried used this:

<?php $my_query = new WP_Query( 'category_name=featured&posts_per_page=1' );
while ( $my_query->have_posts() ) : $my_query->the_post();
$do_not_duplicate = $post->ID; ?>
	<!-- Do stuff... -->
<?php endwhile; ?>

But doesn’t worked for me…

Can you help me?

Thanks!