I've just upgraded the ACL Module from 6.x-1.0 to 6.x-1.1 on a Drupal 6.15 with MySQL 5.1.42 installation. When running "update.php", I see
acl module: 6001
within the "Select versions" fieldset. After clicking the [ Update ] button the following warning appeared:
user warning: Duplicate key name 'nid' query: ALTER TABLE acl_node ADD INDEX nid (nid) in /usr/share/drupal/includes/database.mysql-common.inc on line 434.