Issue #1492188 by Berdir, catch: Fixed Update module creates duplicate queue items.

modules/update/update.module

@@ -809,14 +809,19 @@ function _update_get_cache_multiple($cid_prefix) {
  *
  * @param $cid
  *   Optional cache ID of the record to clear from the private update module
- *   cache. If empty, all records will be cleared from the table.
+ *   cache. If empty, all records will be cleared from the table except
+ *   fetch tasks.
  * @param $wildcard
  *   If $wildcard is TRUE, cache IDs starting with $cid are deleted in
  *   addition to the exact cache ID specified by $cid.
  */
 function _update_cache_clear($cid = NULL, $wildcard = FALSE) {
   if (empty($cid)) {
-    db_truncate('cache_update')->execute();
+    db_delete('cache_update')
+      // Clear everything except fetch task information because these are used
+      // to ensure that the fetch task queue items are not added multiple times.
+      ->condition('cid', 'fetch_task::%', 'NOT LIKE')
+      ->execute();
   }
   else {
     $query = db_delete('cache_update');

modules/update/update.test

@@ -225,6 +225,33 @@ class UpdateCoreTestCase extends UpdateTestHelper {
     $this->assertUniqueText(t('Failed to get available update data for one project.'));
   }
 
+  /**
+   * Tests that exactly one fetch task per project is created and not more.
+   */
+  function testFetchTasks() {
+    $projecta = array(
+      'name' => 'aaa_update_test',
+    );
+    $projectb = array(
+      'name' => 'bbb_update_test',
+    );
+    $queue = DrupalQueue::get('update_fetch_tasks');
+    $this->assertEqual($queue->numberOfItems(), 0, 'Queue is empty');
+    update_create_fetch_task($projecta);
+    $this->assertEqual($queue->numberOfItems(), 1, 'Queue contains one item');
+    update_create_fetch_task($projectb);
+    $this->assertEqual($queue->numberOfItems(), 2, 'Queue contains two items');
+    // Try to add project a again.
+    update_create_fetch_task($projecta);
+    $this->assertEqual($queue->numberOfItems(), 2, 'Queue still contains two items');
+
+    // Clear cache and try again.
+    _update_cache_clear();
+    drupal_static_reset('_update_create_fetch_task');
+    update_create_fetch_task($projecta);
+    $this->assertEqual($queue->numberOfItems(), 2, 'Queue contains two items');
+  }
+
   protected function setSystemInfo7_0() {
     $setting = array(
       '#all' => array(