--- temporary_invitation.module.5.x-1.8	2007-05-17 03:33:56.000000000 +0200
+++ temporary_invitation.module	2007-05-22 14:22:24.000000000 +0200
@@ -396,10 +396,14 @@ function _temporary_invitation_overview(
 function temporary_invitation_manage_validate($form_id, $form_values) {
   // If something is entered at all, validate the e-mail address.
   // (Otherwise, the '#required' property takes care of this.)
-  if (!empty($form_values['invited_email'])) {
-    if ($error = user_validate_mail($form_values['invited_email'])) {
+  $invited_email = $form_values['invited_email'];
+  if (!empty($invited_email)) {
+    if ($error = user_validate_mail($invited_email)) {
       form_set_error('mail', $error);
     }
+    if (db_num_rows(db_query("SELECT mail FROM {users} WHERE LOWER(mail) = LOWER('%s')", $invited_email)) > 0) {
+      form_set_error('mail', t('The e-mail address %email is already registered.', array('%email' => $invited_email)));
+    }
   }
 }