使用update.php出現一些錯誤

最近剛開始學著架drupal，看了蠻多教學的，好像有裝新的模組都要update.php一下，以下是我碰到的問題，不知道要如何解決，謝謝:)

user warning: Duplicate key name 'pid' query: ALTER TABLE comments ADD INDEX pid (pid) in /home/zth/public_html/drupal/includes/database.mysql-common.inc on line 434

The following queries were executed
system module
Update #6047

* UPDATE {blocks} SET cache = -1 WHERE module = 'user' AND delta IN ('0', '1')
* UPDATE {blocks} SET cache = -1 WHERE module = 'system' AND delta = '0'

blogapi module
Update #6000

* No queries

book module
Update #6000

* DROP TABLE {book}
* CREATE TABLE {book} ( `mlid` INT unsigned NOT NULL DEFAULT 0, `nid` INT unsigned NOT NULL DEFAULT 0, `bid` INT unsigned NOT NULL DEFAULT 0, PRIMARY KEY (mlid), UNIQUE KEY nid (nid), INDEX bid (bid) ) /*!40100 DEFAULT CHARACTER SET UTF8 */

comment module
Update #6003

* Failed: ALTER TABLE {comments} ADD INDEX pid (pid)

locale module
Update #6005

* No queries

update module
Update #6000

* No queries

forum module
Update #6000

* No queries