Index: includes/form.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/form.inc,v
retrieving revision 1.174.2.3
diff -u -F^f -r1.174.2.3 form.inc
--- includes/form.inc	29 Jan 2007 21:51:53 -0000	1.174.2.3
+++ includes/form.inc	15 Feb 2007 18:37:25 -0000
@@ -397,6 +397,13 @@ function drupal_validate_form($form_id, 
     }
   }
 
+  if (!$form['#programmed'] && !empty($_SESSION['last_submitted']['hash'])) {
+    if ($_SESSION['last_submitted']['hash'] == md5(serialize($form))) {
+      // This is a repeat submission.
+      drupal_redirect_form(NULL, $_SESSION['last_submitted']['destination']);
+    }
+  }
+
   _form_validate($form, $form_id);
   $validated_forms[$form_id] = TRUE;
 }
@@ -432,6 +439,11 @@ function drupal_submit_form($form_id, $f
       }
     }
   }
+
+  // Successful submit. Hash this form and store the hash in the session.
+  // We'll use this hash later whenever this user submits another form
+  // to make sure no identical forms get submitted twice.
+  $_SESSION['last_submitted'] = array('destination' => $goto, 'hash' => md5(serialize($form)));
   return $goto;
 }