Index: wp-includes/comment.php
===================================================================
--- wp-includes/comment.php	(revision 23961)
+++ wp-includes/comment.php	(working copy)
@@ -671,10 +671,10 @@
 
 	// Simple duplicate check
 	// expected_slashed ($comment_post_ID, $comment_author, $comment_author_email, $comment_content)
-	$dupe = "SELECT comment_ID FROM $wpdb->comments WHERE comment_post_ID = '$comment_post_ID' AND comment_parent = '$comment_parent' AND comment_approved != 'trash' AND ( comment_author = '$comment_author' ";
+	$dupe = $wpdb->prepare( "SELECT comment_ID FROM $wpdb->comments WHERE comment_post_ID = %d AND comment_parent = %s AND comment_approved != 'trash' AND ( comment_author = %s ", wp_unslash( $comment_post_ID ), wp_unslash( $comment_parent ), wp_unslash( $comment_author ) );
 	if ( $comment_author_email )
-		$dupe .= "OR comment_author_email = '$comment_author_email' ";
-	$dupe .= ") AND comment_content = '$comment_content' LIMIT 1";
+		$dupe .= $wpdb->prepare( "OR comment_author_email = %s ", wp_unslash( $comment_author_email ) );
+	$dupe .= $wpdb->prepare( ") AND comment_content = %s LIMIT 1", wp_unslash( $comment_content ) );
 	if ( $wpdb->get_var($dupe) ) {
 		do_action( 'comment_duplicate_trigger', $commentdata );
 		if ( defined('DOING_AJAX') )