Constants should be checked if defined before being set

Some settings like DOMAIN_INSTALL_RULE or DOMAIN_SITE_GRANT need to be modified in the domain.module to take effect. In my opinion, these constants should be overridden in Drupal settings.php, so that we don't need to change the module's source code. If so, we don't need to modify the source whenever the module is upgraded to newer version.

Just modify the domain.module like below:

if (!defined('DOMAIN_INSTALL_RULE')) 
   define('DOMAIN_INSTALL_RULE', TRUE);