Query string in index file duplicating posts

Hi there,

I am trying to order the posts by the date modified rathter than created. While researching i found a snippet of code that you add in the index file but when editing or even adding a new post they are duplicating. Now from more research i think this has something to do with looping but still unsure…

Can someone please advise me where this code has gone wrong?

Thanks.

Code from index.php

<?php get_header(); ?>
<?php get_template_part( 'nav', 'above' ); ?>
//The next line is what i have added in.
<?php  global $query_string; query_posts( $query_string . '&orderby=modified' );?>
<?php while ( have_posts() ) : the_post() ?>
<?php get_template_part( 'entry' ); ?>
<?php get_sidebar(); ?>
<?php endwhile; ?>
<?php get_template_part( 'nav', 'below' ); ?>
<?php get_footer(); ?>

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