index.php

<?php
/*
Plugin Name: Unique WP Query
Plugin URI: https://github.com/firstandthird/unique_wp_query
Description: A plugin for doing unique WP_Query
Author: firstandthird
Version: 1.0.0
// adapted & taken from https://gist.github.com/jameswburke/d0776d742ab74c469cd8af8dacd916fc
 */

class Unique_WP_Query extends WP_Query {
  function __construct($args) {
    // Act as a flag for pre_get_posts
    $args['unique_wp_query'] = true;

    // don't get any pages specified by post__not_in:
    if (key_exists( 'post__not_in', $args)) {
      foreach ($args['post__not_in'] as $key => $id) {
        Unique_WP_Query_Manager::add_post_id($id);
      }
      unset($args['post__not_in']);
    }

    // don't get the current post or page:
    if (is_single()) {
      $id = get_the_ID();
      Unique_WP_Query_Manager::add_post_id($id);
    }

    // Initialize the WP_Query object like normal
    parent::__construct($args);

    // Track used posts, and remove duplicates
    Unique_WP_Query_Manager::process_posts($this->posts, $this->post_count, $this->get('original_posts_per_page'));
  }
}

/**
 * Keeps track of which posts have already appeared and
 * removes them from future Unique_WP_Query objects.
 */
class Unique_WP_Query_Manager {

  /**
   * Keep track of which post_ids have already been used
   * @var array
   */
  public static $used_post_ids = [];

  /**
   * Keeps track of how many used_posts_ids exist
   * @var integer
   */
  public static $used_post_count = 0;

  /**
   * Helper to make sure no duplicate ids are added
   * useful for content outside of the normal query
   */
  public static function add_post_id($id) {
    if (!in_array($id, Unique_WP_Query_Manager::$used_post_ids, true)) {
      Unique_WP_Query_Manager::$used_post_ids[] = $id;
      Unique_WP_Query_Manager::$used_post_count = count(Unique_WP_Query_Manager::$used_post_ids);
    }
  }

  /**
   * Takes the posts and post count of a WP_Query object and
   * ensures that it removes posts that have already been used,
   * and then trims it to the necessary size.
   *
   * @param array &$posts
   * @param integer &$post_count
   */
  public static function process_posts(&$posts, &$post_count, $original_posts_per_page) {

    // Keep track of how many posts are acceptable to return
    $current_accepted_post_count = 0;

    // Make sure we have posts
    if (empty((array) $posts)) {
      return;
    }

    // Loop through all the found posts
    foreach ((array) $posts as $key => $post) {

      // If we have enough acceptable posts, break the loop
      // Otherwise, determine if we've already used this post
      if ($original_posts_per_page > $current_accepted_post_count) {

        // Has this post already been used?
        if (in_array($post->ID, Unique_WP_Query_Manager::$used_post_ids, true)) {
          // Remove from $posts
          unset($posts[$key]);

          // And update count
          $post_count--;
        } else {
          // If not, add it to the list of used ids
          Unique_WP_Query_Manager::$used_post_ids[] = $post->ID;

          // Update how many accepted posts we have
          $current_accepted_post_count++;
        }
      } else {
        // If we have enough acceptable posts, break the foreach
        // This prevents extra results from accidently being added to $used_post_ids
        break;
      }
    }

    // Reindex the array correctly
    $posts = array_values($posts);

    // Update the $used_post_count
    Unique_WP_Query_Manager::$used_post_count = count(Unique_WP_Query_Manager::$used_post_ids);

    // Trim any excess posts and update $post_count
    if (count($posts) > $original_posts_per_page) {

      // Remove extra posts
      $posts = array_slice($posts, 0, $original_posts_per_page);

      // Update post count to our new value
      $post_count = $original_posts_per_page;
    }
  }
}

if (!function_exists('unique_wp_query_pre_get_posts')) {
  /**
   * Increase the posts_per_page value by the number of posts that have
   * already been used. Executes as the last pre_get_posts action.
   */
  function unique_wp_query_pre_get_posts(&$query) {
    if (true === $query->get('unique_wp_query')) {
      $posts_per_page = $query->get('posts_per_page');
      // Increase posts_per_page by the amount of used post_ids
      $query->set('original_posts_per_page', min($posts_per_page, 200));
      $query->set('posts_per_page', min($posts_per_page + Unique_WP_Query_Manager::$used_post_count, 200));
    }
  }

  add_action('pre_get_posts', 'unique_wp_query_pre_get_posts', PHP_INT_MAX);
}