Update postdata without creating duplicate on wp_insert_post from RSS feed

You can save AdvertID with your post and use it to check if the job was imported before.

/**
 * Gets a job by AdvertId.
 * 
 * @param   string          $advert_id  The AdvertId.
 * @return  WP_Post|bool                The job or <false> if no job is found.
 */
function get_job_by_advert_id( $advert_id ) {
    $args = array(
        'post_type' => 'jobs',
        'meta_query' => array(
            'key' => 'advert_id',
            'value' => $advert_id,  
        ),
    );
    $posts = get_posts( $args );

    if (empty($posts)) {
        return false;
    }

    return $posts[0];
}

// Check if the job was imported before.
$job = get_job_by_advert_id( $item->AdvertId );

if ( $job ) {

    // The job was imported before, update it.
    $args = array(
        'ID' => $job->ID,
        'post_content'   => $item->description,
        'post_date'      => $item_date,
        'post_title'     => $item_title,
    );

    $post_id = wp_update_post( $args ); 

} else {

    // The job is new. Insert it.
    $args = array(
      'post_content'   => $item->description,
      'post_date'      => $item_date,
      'post_title'     => $item_title,
      'post_status'    => 'publish',
      'post_type'      => 'jobs',
    );
    $post_id = wp_insert_post($args);   

    // Save the AdvertId for the next import.
    add_post_meta( $post_id, 'advert_id', $item->AdvertId, true);

}