memcache.db.inc - Cannot redeclare cache_get()

When using

include_once('./includes/cache.inc');
include_once('./sites/all/modules/memcache/memcache.inc');
$conf['cache_default_class'] = 'MemCacheDrupal';

memcache is working fine.

However, when I try to use memcache.db.inc I get the following error:

Fatal error: Cannot redeclare cache_get() (previously declared in /var/www/html/drupal7/includes/cache.inc:49) in /var/www/html/drupal7/sites/all/modules/memcache/memcache.db.inc on line 77

Regards.