--- wp-register.php.original	2005-07-02 05:00:17.000000000 +0300
+++ wp-register.php	2005-07-02 05:01:14.000000000 +0300
@@ -47,6 +47,12 @@
 		die (__('<strong>ERROR</strong>: This username is already registered, please choose another one.'));
 	}
 
+	/* checking the email isn't already used by another user */
+	$result = $wpdb->get_results("SELECT user_email FROM $wpdb->users WHERE user_email = '$user_email'");
+	if (count($result) >= 1) {
+		die (__('<strong>ERROR</strong>: This email address is already registered, please choose another one.'));
+	}
+
 	$user_ip = $_SERVER['REMOTE_ADDR'] ;
 
 	$user_browser = $wpdb->escape($_SERVER['HTTP_USER_AGENT']);