title: Making only one task run at a time in celerybeat
body: <p>I have a task which I execute once a minute using celerybeat. It works fine. Sometimes though, the task takes a few seconds more than a minute to run because of which two instances of the task run. This leads to some race conditions that mess things up.</p>
  
  <p>I can (and probably should) fix my task to work properly but I wanted to know if celery has any builtin ways to ensure this. My cursory Google searches and RTFMs yielded no results.</p>