Well nowadays their are lots of complaint about the well known plugin know YARPP or you can say Yet Another Related Posts Plugin that it is showing results twice on the same page
Well according to author of the plugin, it has been solved, as the latest YARPP plugin still shows twice results it is because of calling the function in a improper way for example :
<?php echo related_posts(); ?>
well the solution to problem is simple just remove the echo from above code so it will become :
<?php related_posts(); ?>
Well your YARPP plugin is fixed.
Don't have YARPP plugin well you can download it from here