Index: modules/trigger/trigger.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/trigger/trigger.module,v
retrieving revision 1.1
diff -u -p -r1.1 trigger.module
--- modules/trigger/trigger.module	11 Sep 2007 14:50:05 -0000	1.1
+++ modules/trigger/trigger.module	26 Sep 2007 01:34:15 -0000
@@ -187,6 +187,10 @@ function _trigger_get_hook_aids($hook, $
  * @param $description
  *   A plain English description of what this hook operation does.
  * @return
+ *
+ * @ingoup forms
+ * @see trigger_assign_form_validate()
+ * @see trigger_assign_form_submit()
  */
 function trigger_assign_form($form_state, $hook, $op, $description) {
   $form['hook'] = array(
@@ -197,7 +201,8 @@ function trigger_assign_form($form_state
     '#type' => 'hidden',
     '#value' => $op,
   );
-  // All of these forms use the same #submit function.
+  // All of these forms use the same validate and submit functions.
+  $form['#validate'][] = 'trigger_assign_form_validate';
   $form['#submit'][] = 'trigger_assign_form_submit';
 
   $options = array();
@@ -253,7 +258,24 @@ function trigger_assign_form($form_state
   }
   return $form;
 }
+/**
+ * Validation function for trigger_assign_form().
+ *
+ * Makes sure that the user is not re-assigning an action to an event.
+ */
+function trigger_assign_form_validate($form, $form_state) {
+  $form_values = $form_state['values'];
+  if (!empty($form_values['aid'])) {
+    $aid = actions_function_lookup($form_values['aid']);
+    if (db_result(db_query("SELECT aid FROM {trigger_assignments} WHERE hook = '%s' AND op = '%s' AND aid = '%s'", $form_values['hook'], $form_values['operation'], $aid))) {
+      form_set_error($form_values['operation'], t('The action you choose is already assigned to that trigger.'));
+    }
+  }
+}
 
+/**
+ * Submit function for trigger_assign_form().
+ */
 function trigger_assign_form_submit($form, $form_state) {
   $form_values = $form_state['values'];
 
Index: modules/trigger/trigger.schema
===================================================================
RCS file: /cvs/drupal/drupal/modules/trigger/trigger.schema,v
retrieving revision 1.1
diff -u -p -r1.1 trigger.schema
--- modules/trigger/trigger.schema	11 Sep 2007 14:50:05 -0000	1.1
+++ modules/trigger/trigger.schema	26 Sep 2007 01:34:15 -0000
@@ -9,8 +9,7 @@ function trigger_schema() {
       'aid' => array('type' => 'varchar', 'length' => 255, 'not null' => TRUE, 'default' => ''),
       'weight' => array('type' => 'int', 'not null' => TRUE, 'default' => 0),
     ),
-    'index keys' => array(
-      'hook_op' => array('hook', 'op'))
+    'primary key' => array('hook', 'op', 'aid'),
   );
   return $schema;
 }