**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
ti1b = dr1.get_task_instance(task_id='dummy_b', session=session)
        ti1b.refresh_from_db(session=session)
        assert ti1b.state == State.SUCCESS
        ti2 = dr2.get_task_instance(task_id='dummy', session=session)
        ti2.refresh_from_db(session=session)
        assert ti2.state == State.SCHEDULED
        ti3.refresh_from_db(session=session)
        assert ti3.state == State.NONE
        assert ti3.start_date is not None
        assert ti3.end_date is None
        assert ti3.duration is None
        dr1.refresh_from_db(session=session)
        dr1.state = State.FAILED
        
        session.merge(dr1)
        session.commit()
        scheduler._change_state_for_tis_without_dagrun(
            old_states=[State.SCHEDULED, State.QUEUED], new_state=State.NONE, session=session
        )
        
        session.expunge_all()
        ti1a.refresh_from_db(session=session)
        assert ti1a.state == State.NONE
        
        ti1b.refresh_from_db(session=session)
        assert ti1b.state == State.SUCCESS
        
        ti2.refresh_from_db(session=session)
        assert ti2.state == State.SCHEDULED
    def test_change_state_for_tasks_failed_to_execute(self):