För att förbättra Blackbarns förslag, prova detta (efter att ha säkerhetskopierat db):
global $wpdb;
$comments = $wpdb->get_results("SELECT * FROM ".$wpdb->prefix."_comments"
." ORDER BY comment_post_ID, comment_content");
$prev = NULL;
foreach($comments as $comment) {
if ($prev && $prev->comment_content == $comment->comment_content
&& $prev->comment_post_ID == $comment->comment_post_ID ) { // add maybe other rules here
$wpdb->query("DELETE FROM ".$wpdb->prefix."_comments WHERE comment_ID = ".$comment->comment_ID);
}
else
$prev = $comment;
}