PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'PRIMARY'

after inserting a second rewrite rule:

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'PRIMARY': INSERT INTO {feeds_tamper} (id, importer, source, plugin_id, settings, weight, description) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6); Array ( [:db_insert_placeholder_0] => [:db_insert_placeholder_1] => proficlass [:db_insert_placeholder_2] => parent_id [:db_insert_placeholder_3] => rewrite [:db_insert_placeholder_4] => a:1:{s:4:"text";s:49:"Proficlass-[klassifikationssystem_id]-[parent_id]";} [:db_insert_placeholder_5] => 1 [:db_insert_placeholder_6] => Proficlass-[klassifikationssystem_id]-[parent_id] ) in drupal_write_record() (Zeile 6888 von /home/xl/workspace/stipro/includes/common.inc).