**Apache Airflow version**: 2.0.0rc3

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**: Linux
- **OS** (e.g. from /etc/os-release): Ubuntu
- **Kernel** (e.g. `uname -a`): 
- **Install tools**:
- **Others**:

**What happened**:

After shutting down the scheduler, while tasks were in running state, trying to restart the scheduler results in pk violations..

```[2020-12-15 22:43:29,673] {scheduler_job.py:1293} ERROR - Exception when executing SchedulerJob._run_scheduler_loop
Traceback (most recent call last):
  File "/home/jcoder/git/airflow_2.0/pyenv/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1277, in _execute_context
    cursor, statement, parameters, context
  File "/home/jcoder/git/airflow_2.0/pyenv/lib/python3.7/site-packages/sqlalchemy/engine/default.py", line 593, in do_execute
    cursor.execute(statement, parameters)
psycopg2.errors.UniqueViolation: duplicate key value violates unique constraint "dag_run_dag_id_run_id_key"
DETAIL:  Key (dag_id, run_id)=(example_task_group, scheduled__2020-12-14T04:31:00+00:00) already exists.
```


**What you expected to happen**:

Scheduler restarts and picks up where it left off.

**How to reproduce it**:

Set example dag ( I used task_group) to schedule_interval `* * * * *` and start the scheduler and let it run for a few minutes. 
Shut down the scheduler
Attempt to restart the scheduler

**Anything else we need to know**:
I came across this doing testing using the LocalExecutor in a virtual env. If no else is able to reproduce it, I'll try again in a clean virtual env.

tests/jobs/test_scheduler_job.py
job = SchedulerJob(None, heartrate=10, state=State.RUNNING)
        assert job.is_alive()
        job.latest_heartbeat = timezone.utcnow() - datetime.timedelta(seconds=20)
        assert job.is_alive()
        job.latest_heartbeat = timezone.utcnow() - datetime.timedelta(seconds=31)
        assert not job.is_alive()
        
        
        job.latest_heartbeat = timezone.utcnow() - datetime.timedelta(days=1)
        assert not job.is_alive()
        job.state = State.SUCCESS
        job.latest_heartbeat = timezone.utcnow() - datetime.timedelta(seconds=10)
        assert not job.is_alive(), "Completed jobs even with recent heartbeat should not be alive"
    def run_single_scheduler_loop_with_no_dags(self, dags_folder):
        """
        Utility function that runs a single scheduler loop without actually
        changing/scheduling any dags. This is useful to simulate the other side effects of
        running a scheduler loop, e.g. to see what parse errors there are in the
        dags_folder.
        :param dags_folder: the directory to traverse
        :type dags_folder: str
        """
        scheduler = SchedulerJob(
            executor=self.null_exec, num_times_parse_dags=1, subdir=os.path.join(dags_folder)
        )
        scheduler.heartrate = 0
        scheduler.run()
    @pytest.mark.quarantined
    def test_no_orphan_process_will_be_left(self):