title: Airflow: Only allow one instance of task
body: <p><a href="https://i.stack.imgur.com/XPiJ3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/XPiJ3.png" alt="DAG Tree view"></a></p>
  
  <p>Is there a way specify that a task can only run once concurrently? So in the tree above where DAG concurrency is 4, Airflow will start task 4 instead of a second instance of task 2?</p>
  
  <p>This DAG is a little special because there is no order between the tasks. These tasks are independent but related in purpose and therefore kept in one DAG so as to new create an excessive number of single task DAGs.</p>
  
  <p><code>max_active_runs</code> is 2 and <code>dag_concurrency</code> is 4. I would like it start all 4 tasks and only start a task in next if same task in previous run is done.</p>