**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
dr = dag.create_dagrun(
                run_type=DagRunType.SCHEDULED,
                execution_date=dag.following_schedule(dr.execution_date),
                state=State.RUNNING,
            )
        ex_date = dr.execution_date
        for tid, state in expected_task_states.items():
            if state != State.FAILED:
                continue
            self.null_exec.mock_task_fail(dag_id, tid, ex_date)
        try:
            dag = DagBag().get_dag(dag.dag_id)
            assert not isinstance(dag, SerializedDAG)
            
            dag.run(start_date=ex_date, end_date=ex_date, executor=self.null_exec, **run_kwargs)
        except AirflowException:
            pass
        
        for task_id, expected_state in expected_task_states.items():
            task = dag.get_task(task_id)
            ti = TaskInstance(task, ex_date)
            ti.refresh_from_db()
            assert ti.state == expected_state
        
        dr = DagRun.find(dag_id=dag_id, execution_date=ex_date)
        dr = dr[0]
        dr.dag = dag
        assert dr.state == dagrun_state
    def test_dagrun_fail(self):