Related WordPress custom posts and posts loop that excludes duplicates

related-posts-loop.php

<?php
// Loop for related posts and custom posts by category 

// Get all categories for current post
$categories = get_the_category();

// Assign current post ID to var to be excluded in loop
$exclude_post = $post->ID;

// Array to store post IDs that will be used to stop duplicates
// SRC: http://wpengineer.com/1719/filter-duplicate-posts-in-the-loop/ 
$do_not_duplicate = array();

if ($categories) { ?>

  <?php foreach ( $categories as $category ) {

    $category_id = $category->cat_ID;

    $args = array (
      'cat' => $category_id,
      // Include custom post types
      'post_type' => array('post', 'custom-post-type' ),
      'posts_per_page'=>12,
      'post_status' => 'publish',
      // Don't show the current post
      'post__not_in' => array($exclude_post),
    );

    // Set up a variable to call new WP_Query
    $loop = new WP_Query( $args );

    // Start the loop
    while ($loop->have_posts()) : $loop->the_post(); 

      // Check var $do_not_duplicate array for duplicate IDs
      if ( !in_array( $post->ID, $do_not_duplicate ) ) {

        // Add post ID to var $do_not_duplicate array
        $do_not_duplicate[] = $post->ID; ?>   

        <a href="<?php the_permalink() ?>"><h4><?php the_title() ?></h4></a>

    <?php 
    // End if - check var $do_not_duplicate array for duplicates 
    }

    // End post loop
    endwhile;

  } // End foreach category

} // End if categories

// Rewind the posts so we can have more loops in the page
rewind_posts() ?>