Multiple loops without repeating content

Question

I have a second loop on a page where I want to display posts from the same category as the present post but excluding the present post.

I have a loop that displays all the posts from the same category as the current post, but does not exclude the current post.

  <?php
  $project_category = wp_get_post_categories($post->ID); 
  $postid = $post->ID;
  ?>

  <?php
  $the_query = new WP_Query( array(
      'category__in' => $project_category,
      'posts_per_page' => -1,
      'post__not_in' => $postid,
  ) );
  ?>

  <?php                                                      
  $loop = new WP_Query( $the_query );
  while ( $loop->have_posts() ) : $loop->the_post();  
  ?>

Answer 1

As explained in Codex

post__not_in (array) - use post ids. Specify post NOT to retrieve.

post__not_in argument have to be passed as array.

Change your query to:

$the_query = new WP_Query( array(
    'category__in'   => $project_category,
    'posts_per_page' => -1,
    'post__not_in'   => array( $postid ),
) );

And it should work.