Small script to delete duplicate post comments from a Wordpress database. Backup first bud.

remove_doop_comments.php

<?php
# First select all comments
$query = "SELECT `comment_ID`, `comment_post_ID`, `comment_content` FROM ".$wpdb->comments." WHERE 1";
$comments = $wpdb->get_results($query);

# Array to hold keeper comment IDs so we don't delete them if there are doops
$keeper_comments = array();

# Now check if each comment has any matching comments from the same post
foreach ($comments as $comment) {
  $query = "SELECT `comment_ID` FROM ".$wpdb->comments." WHERE `comment_ID` != ".$comment->comment_ID." AND `comment_post_ID` = ".$comment->comment_post_ID." AND `comment_content` = '".addslashes($comment->comment_content)."'";
	$matching_comments = $wpdb->get_results($query);
	if ($wpdb->num_rows > 0) {
		foreach ($matching_comments as $matching_comment) {
			if (!in_array($matching_comment->comment_ID, $keeper_comments)) {
				$wpdb->query("DELETE FROM ".$wpdb->comments." WHERE `comment_ID` = ".$matching_comment->comment_ID);
				$wpdb->query("UPDATE ".$wpdb->posts." SET `comment_count` = `comment_count` - 1 WHERE `comment_ID` = ".$matching_comment->comment_ID);
			}
		}
		$keeper_comments[] = $comment->comment_ID;
	}
}

Does it still work?