**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
)
        ti1, ti2 = dr1.get_task_instances(session=session)
        dr1.state = State.RUNNING
        ti1.state = State.SCHEDULED
        ti1.queued_by_job_id = old_job.id
        session.merge(dr1)
        session.merge(ti1)
        ti2.state = State.SCHEDULED
        ti2.queued_by_job_id = scheduler_job.id
        session.merge(ti2)
        session.flush()
        num_reset_tis = scheduler_job.adopt_or_reset_orphaned_tasks(session=session)
        assert 1 == num_reset_tis
        session.refresh(ti1)
        assert ti1.state is None
        session.refresh(ti2)
        assert State.SCHEDULED == ti2.state
        session.rollback()
    def test_send_sla_callbacks_to_processor_sla_disabled(self):
        """Test SLA Callbacks are not sent when check_slas is False"""
        dag_id = 'test_send_sla_callbacks_to_processor_sla_disabled'
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE, schedule_interval='@daily')
        DummyOperator(task_id='task1', dag=dag)
        with patch.object(settings, "CHECK_SLAS", False):
            scheduler_job = SchedulerJob(subdir=os.devnull)
            mock_agent = mock.MagicMock()
            scheduler_job.processor_agent = mock_agent
            scheduler_job._send_sla_callbacks_to_processor(dag)
            scheduler_job.processor_agent.send_sla_callback_request_to_execute.assert_not_called()
    def test_send_sla_callbacks_to_processor_sla_no_task_slas(self):