**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
"""
        Test that scheduler._create_dag_runs does not raise an error when the DAG does not exist
        in serialized_dag table
        """
        dag = DAG(dag_id='test_scheduler_create_dag_runs_does_not_raise_error', start_date=DEFAULT_DATE)
        DummyOperator(
            task_id='dummy',
            dag=dag,
        )
        dagbag = DagBag(
            dag_folder=os.devnull,
            include_examples=False,
            read_dags_from_db=False,
        )
        dagbag.bag_dag(dag=dag, root_dag=dag)
        
        DAG.bulk_write_to_db(dagbag.dags.values())
        dag_model = DagModel.get_dagmodel(dag.dag_id)
        scheduler = SchedulerJob(subdir=os.devnull, executor=self.null_exec)
        scheduler.processor_agent = mock.MagicMock()
        with create_session() as session, self.assertLogs(
            'airflow.jobs.scheduler_job', level="ERROR"
        ) as log_output:
            scheduler._create_dag_runs([dag_model], session)
            assert (
                "airflow.exceptions.SerializedDagNotFound: DAG "
                "'test_scheduler_create_dag_runs_does_not_raise_error' not found in serialized_dag table"
            ) in log_output.output[0]
    def test_bulk_write_to_db_external_trigger_dont_skip_scheduled_run(self):