Index: guestbook.module
===================================================================
RCS file: /cvs/drupal-contrib/contributions/modules/guestbook/guestbook.module,v
retrieving revision 1.17
diff -u -p -r1.17 guestbook.module
--- guestbook.module	9 Sep 2008 18:17:23 -0000	1.17
+++ guestbook.module	9 Sep 2008 18:27:36 -0000
@@ -479,6 +479,18 @@ function guestbook_form_entry_form($form
   return $form;
 }
 
+/**
+ * Form validation function for guestbook_form_entry_form.
+ */
+function guestbook_form_entry_form_validate($form, &$form_state) {
+  // Check whether name of anonymous user is an registered user's name.
+  if (isset($form_state['values']['anonname'])) {
+    if ($existing = user_load(array('name' => $form_state['values']['anonname']))) {
+      form_set_error('anonname', t('%name is a registered user name. Please enter a different name.', array('%name' => $form_state['values']['anonname'])));
+    }
+  }
+}
+
 function guestbook_form_entry_form_submit($form, &$form_state) {
   global $user;