remove duplicates from loop

in my wordpress site, i have categories like this :
Parent Category 1
– Sub Category 1
– Title
– Title
– Sub Category 2
– Title
– Title
Parent Category 2
– Sub Category 1
– Title
– Title
– Sub Category 2
– Title
– Title
Category 3
– Title
– Title

I want to display post, for each term as term > display post
but i don’t want to display Parent Category , i want a loop to be like this:
– Sub Category 1
– Title
– Title
– Sub Category 2
– Title
– Title

sometimes posts are associated with 2 category, how to remove duplicates ?

<?php
$do_not_duplicate = array();
//get all categories then display all posts in each term
$taxonomy = 'category';
$param_type = 'category__in';
$term_args=array(

);
$terms = get_terms($taxonomy,$term_args);
if ($terms) {
  foreach( $terms as $term ) {
    $args=array(
      "$param_type" => array($term->term_id),
      'connected_type' => 'posts_to_invitations',
      'connected_items' => get_queried_object(),
      'nopaging' => true,
      'post_type' => 'post',
      'post_status' => 'publish,draft',
      'posts_per_page' => -1,
      'post__not_in'     => $do_not_duplicate,
      'caller_get_posts'=> 1
      );
    $my_query = null;
    $my_query = new WP_Query($args);
    if( $my_query->have_posts() ) {
	$do_not_duplicate[] = get_the_ID();	?>
<?php
      while ($my_query->have_posts()) : $my_query->the_post(); ?>
		<tr>
				<td><?php include (TEMPLATEPATH . '/contactfullname.php'); ?></td>
				<td><?php echo get_post_meta($post->ID, 'ecpt_position', true); ?></td>
				<td><?php echo p2p_get_meta( get_post()->p2p_id, 'table', true ); ?></td>
				<td><?php echo ' ' .$term->name;?></td>
				<td><?php echo p2p_get_meta( get_post()->p2p_id, 'confirmation', true ); ?></td>
		</tr>
       <?php
      endwhile;
      ?>
<?php
    }
  }
}
wp_reset_query();  // Restore global post data stomped by the_post().
?>