When running drush from Terminal in OSX ensure that the directory path you are in is case-correct. For example, if you run drush in ~/sites/drupal when your path should be ~/Sites/drupal you will get this error:

Fatal error: Cannot redeclare system_help() (previously declared in /path/to/home/sites/drupal/modules/system/system.module:47) in /path/to/home/Sites/drupal/modules/system/system.module on line 106

Terminal in MacOS will not force you to use the correct case when you cd into your drupal directory.

Source: http://drupal.org/node/644126