Query multiple/duplicate meta_key values

SELECT $wpdb->posts. *
FROM $wpdb->posts, $wpdb->postmeta
WHERE $wpdb->posts.ID = $wpdb->postmeta.post_id
AND $wpdb->postmeta.meta_key = 'productcode'
AND $wpdb->posts.post_status = 'publish'
AND $wpdb->posts.post_type = 'post'
AND $wpdb->posts.post_date < NOW( )
AND $wpdb->postmeta.meta_value = $wpdb->postmeta.meta_value
GROUP BY $wpdb->postmeta.meta_value
HAVING count( * ) >1
ORDER BY $wpdb->posts.post_date DESC