Exclude Duplicate Posts in this SELECT Query

I have this custom select query that pulls a specified number of posts from a specific category and displays them in the order of the latest comments. The problem is that if a post has both of the latest comments, it will get shown twice. I would like it to somehow detect duplicate posts and exclude them, showing only the most recent comment.

$sql =
"SELECT DISTINCT ID, post_title, post_password, comment_ID, comment_post_ID,
    comment_author, comment_date, comment_approved, comment_type,comment_author_url,
    SUBSTRING(comment_content,1,100) AS com_excerpt
FROM $wpdb->term_taxonomy as t1, $wpdb->posts, $wpdb->term_relationships as r1, $wpdb->comments
WHERE comment_approved = '1'
   AND comment_type = ''
   AND ID = comment_post_ID
   AND post_password = ''
   AND ID = r1.object_id
   AND r1.term_taxonomy_id = t1.term_taxonomy_id
   AND t1.taxonomy = 'category'
   AND t1.term_id IN ($inlist)
ORDER BY comment_date DESC LIMIT 4";

Any ideas?