CiviCRM crashes adding registered drupal user to CiviEvent.

Here are the failures for two different users.

Drupal 5.1
PHP 5.2.1
MySQL 5.0.37
Mac OS X

Error Details:

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -5
[message] => DB Error: already exists
[mode] => 16
[debug_info] => INSERT INTO civicrm_location (entity_table , entity_id , location_type_id , is_primary ) VALUES ('civicrm_contact' , 115 , 4 , 0 ) [nativecode=1062 ** Duplicate entry 'civicrm_contact-115-4' for key 2]
[type] => DB_Error
[user_info] => INSERT INTO civicrm_location (entity_table , entity_id , location_type_id , is_primary ) VALUES ('civicrm_contact' , 115 , 4 , 0 ) [nativecode=1062 ** Duplicate entry 'civicrm_contact-115-4' for key 2]
[to_string] => [db_error: message="DB Error: already exists" code=-5 mode=callback callback=CRM_Core_Error::handle prefix="" info="INSERT INTO civicrm_location (entity_table , entity_id , location_type_id , is_primary ) VALUES ('civicrm_contact' , 115 , 4 , 0 ) [nativecode=1062 ** Duplicate entry 'civicrm_contact-115-4' for key 2]"]
)

Error Details:

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -5
[message] => DB Error: already exists
[mode] => 16
[debug_info] => INSERT INTO civicrm_location (entity_table , entity_id , location_type_id , is_primary ) VALUES ('civicrm_contact' , 126 , 4 , 0 ) [nativecode=1062 ** Duplicate entry 'civicrm_contact-126-4' for key 2]
[type] => DB_Error
[user_info] => INSERT INTO civicrm_location (entity_table , entity_id , location_type_id , is_primary ) VALUES ('civicrm_contact' , 126 , 4 , 0 ) [nativecode=1062 ** Duplicate entry 'civicrm_contact-126-4' for key 2]
[to_string] => [db_error: message="DB Error: already exists" code=-5 mode=callback callback=CRM_Core_Error::handle prefix="" info="INSERT INTO civicrm_location (entity_table , entity_id , location_type_id , is_primary ) VALUES ('civicrm_contact' , 126 , 4 , 0 ) [nativecode=1062 ** Duplicate entry 'civicrm_contact-126-4' for key 2]"]
)