title: How not to start same task and wait until it is finished with celery beat
body: <p>I have scheduled task with celery beat to run every 3 hours:</p>
  
  <pre><code>'sync_stuff': {
      'task': 'celery_tasks.sync_stuff',
      'schedule': crontab(hour='*/3')
  }
  </code></pre>
  
  <p>Sometimes it takes longer than 3 hours to finish the task and I want to ensure that celery does not schedule and run the task again while the old instance is still running.</p>
  
  <p>Is there a way to do that just with celery or celerybeat settings?</p>