WordPress related custom posts and posts loop that excludes duplicates

related-posts.php

<?php
// Loop for related posts and custom posts by 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();

// Get all the categories for the current post
$post_categories = wp_get_post_categories( $post->ID );
// This prints the values of the array
// print_r(array_values($post_categories));

$args = array (
  'category__in' => $post_categories,
  // Include custom post types
  'post_type' => array('post', 'admin_post', 'article', 'event' ),
  'posts_per_page' => 12,
  'post_status' => 'publish',
  // Don't show the current post
  'post__not_in' => array($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; 

    // All post preview markup is included here 
    get_template_part( 'modules/preview' );

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

// End post loop
endwhile;  

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