**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
execution_date=DEFAULT_DATE,
            start_date=DEFAULT_DATE,
            session=session,
        )
        ti1a = dr1.get_task_instance(task_id='dummy', session=session)
        ti1a.state = State.SCHEDULED
        ti1b = dr1.get_task_instance(task_id='dummy_b', session=session)
        ti1b.state = State.SUCCESS
        session.commit()
        ti2 = dr2.get_task_instance(task_id='dummy', session=session)
        ti2.state = State.SCHEDULED
        session.commit()
        ti3 = TaskInstance(dag3.get_task('dummy'), DEFAULT_DATE)
        ti3.state = State.SCHEDULED
        session.merge(ti3)
        session.commit()
        dagbag = DagBag("/dev/null", include_examples=False, read_dags_from_db=False)
        dagbag.bag_dag(dag1, root_dag=dag1)
        dagbag.bag_dag(dag2, root_dag=dag2)
        dagbag.bag_dag(dag3, root_dag=dag3)
        dagbag.sync_to_db(session)
        session.commit()
        scheduler = SchedulerJob(num_runs=0)
        scheduler.dagbag.collect_dags_from_db()
        scheduler._change_state_for_tis_without_dagrun(
            old_states=[State.SCHEDULED, State.QUEUED], new_state=State.NONE, session=session
        )
        ti1a = dr1.get_task_instance(task_id='dummy', session=session)
        ti1a.refresh_from_db(session=session)
        assert ti1a.state == State.SCHEDULED