Index: handlers/views_handler_filter.inc
===================================================================
RCS file: /cvs/drupal-contrib/contributions/modules/views/handlers/views_handler_filter.inc,v
retrieving revision 1.6.2.5
diff -u -p -r1.6.2.5 views_handler_filter.inc
--- handlers/views_handler_filter.inc	26 Jun 2009 00:23:37 -0000	1.6.2.5
+++ handlers/views_handler_filter.inc	20 Sep 2009 18:57:13 -0000
@@ -271,6 +271,10 @@ class views_handler_filter extends views
     if (!empty($form_state['values']['options']['expose']['identifier']) && $form_state['values']['options']['expose']['identifier'] == 'value') {
       form_error($form['expose']['identifier'], t('This identifier is not allowed.'));
     }
+    
+    if (!$this->view->display_handler->is_identifier_unique($form_state['id'], $form_state['values']['options']['expose']['identifier'])) {
+      form_error($form['expose']['identifier'], t('This identifier is used by another handler.'));
+    }
   }
 
   /**
Index: plugins/views_plugin_display.inc
===================================================================
RCS file: /cvs/drupal-contrib/contributions/modules/views/plugins/views_plugin_display.inc,v
retrieving revision 1.20.2.7
diff -u -p -r1.20.2.7 views_plugin_display.inc
--- plugins/views_plugin_display.inc	15 Sep 2009 19:41:50 -0000	1.20.2.7
+++ plugins/views_plugin_display.inc	20 Sep 2009 18:57:19 -0000
@@ -1840,6 +1840,22 @@ class views_plugin_display extends views
   }
 
   /**
+   * Check if the provided identifier is unique.
+   */ 
+  function is_identifier_unique($id, $identifier) {   
+    foreach (views_object_types() as $type => $info) {
+      foreach ($this->get_handlers($type) as $key => $handler) {
+        if ($handler->can_expose() && $handler->is_exposed()) {
+          if ($id != $key && $identifier == $handler->options['expose']['identifier']) {
+            return FALSE;
+          }
+        }
+      }
+    }
+    return TRUE;
+  }
+
+  /**
    * Provide the block system with any exposed widget blocks for this display.
    */
   function get_special_blocks() {