**Apache Airflow version**:
2.0.1

**Kubernetes version**:
Server Version: version.Info{Major:"1", Minor:"18+", GitVersion:"v1.18.9-eks-d1db3c", GitCommit:"d1db3c46e55f95d6a7d3e5578689371318f95ff9", GitTreeState:"clean", BuildDate:"2020-10-20T22:18:07Z", GoVersion:"go1.13.15", Compiler:"gc", Platform:"linux/amd64"}

**What happened**:
Clearing a failed subdag task with Downstream+Recursive does not automatically set the state of the parent dag to 'running' so that the downstream parent tasks can execute.

The work around is to manually set the state of the parent dag to running after clearing the subdag task

**What you expected to happen**:
With airflow version 1.10.4 the parent dag was automatically set to 'running' for this same scenario

**How to reproduce it**:
- Clear a failed subdag task selecting option for Downstream+Recursive
- See that all down stream tasks in the subdag as well as the parent dag have been cleared
- See that the parent dag is left in 'failed' state. 

airflow/models/dag.py
raise DuplicateTaskIdFound(f"Task id '{task.task_id}' has already been added to the DAG")
        else:
            self.task_dict[task.task_id] = task
            task.dag = self
            
            self._task_group.used_group_ids.add(task.task_id)
        self.task_count = len(self.task_dict)
    def add_tasks(self, tasks):
        """
        Add a list of tasks to the DAG
        :param tasks: a lit of tasks you want to add
        :type tasks: list of tasks
        """
        for task in tasks:
            self.add_task(task)
    def run(
        self,
        start_date=None,
        end_date=None,
        mark_success=False,
        local=False,
        executor=None,
        donot_pickle=conf.getboolean('core', 'donot_pickle'),
        ignore_task_deps=False,
        ignore_first_depends_on_past=True,
        pool=None,
        delay_on_limit_secs=1.0,
        verbose=False,
        conf=None,
        rerun_failed_tasks=False,