How to find what is causing a slug-2?

Step 1: Get all the available post types.
This is done with the following query

global $wpdb;
$query = "SELECT DISTINCT post_type FROM {$wpdb->posts}";
$results = $wpdb->get_results($query);
foreach ($results as $result) {
    echo "'" . $result->post_type . "',";
}

Step 2: Use WP_Query to find the instance with the original slug:

$slug = 'support';
$args = array(
    'name' => $slug,
    'post_type' => array('list', 'from', 'step', 'one'), 
    'post_status' => array('publish', 'pending', 'draft', 'private', 'trash', 'future'),
    'posts_per_page' => -1,
);

// Perform the query
$query = new WP_Query($args);

$posts = $query->posts;
foreach($posts as $post){
    var_dump($post);
}

In my case, it was a nav_menu_item post_type