When doing the authentication via an external db with custom fields users that change their infos in the external DB result in duplicate entries in the custom fields.

SOLUTION
FILE lib/moodlelib.php ($Id: moodlelib.php,v 1.960.2.132 2009/05/15 07:22:23 skodak Exp $)
function: update_user_record
action: before actually inserting the new record, deletes the old (line 2977)

delete_records('user_info_data', 'userid', $oldinfo->id, 'fieldid', $infofield->id);

action: adding a unique index on table user_info_data with userid and fieldid (ahem... I did it straight in the DB ) to prevent duplicate entries.