首先，您需要创建以下函数：

function checkEmailAlreadyExists( $message_id ) {
    $posts_with_meta = get_posts( array(
        \'posts_per_page\' => 1, // we only want to check if any exists, so don\'t need to get all of them
        \'post_type\'      => \'my-custom-post-type\',
        \'meta_key\'       => \'ticket_id\',
        \'meta_value\'     => $message_id,
        \'fields\'         => \'ids\', // we don\'t need it\'s content, etc.
    ) );

    return ( count( $posts_with_meta ) ) ? true : false;    
}

$total = $emails->total_msg(); 
for ( $j=1; $j <= $total; $j++ ) {
    $mail = $emails->get( $j );
    if ( !empty( $mail[\'header\']->message_id ) && !checkEmailAlreadyExists( $mail[\'header\']->message_id )  ) {
        $post_array = array( 
            \'post_content\'  => $mail[\'body\'],
            \'post_title\'    => $mail[\'header\']->subject,
            \'post_type\'     => \'my-custom-post-type\',
            \'post_status\'   => \'publish\',
            \'meta_input\'    => array(
                \'from\'       => $mail[\'header\']->fromaddress,
                \'email_date\' => $mail[\'header\']->Date,
                \'ticket_id\'  => $mail[\'header\']->message_id,
            ),
        );
        wp_insert_post( $post_array );
    }
}