Cannot declare class CRM_Core_ClassLoader, error

Question

For some reason, I'm getting an error message: Fatal error: Cannot declare class CRM_Core_ClassLoader, because the name is already in use in /home3/isrcus/public_html/sites/all/modules/civicrm/CRM/Core/ClassLoader.php on line 36

Even though, there is only one instance of CiviCRM on the site, and I have the following code:

  require_once 'CRM/Core/ClassLoader.php';
  CRM_Core_ClassLoader::singleton()->register();

What could be causing the error??