**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.

airflow/jobs/scheduler_job.py
query = (
            session.query(models.TaskInstance)
            .outerjoin(models.TaskInstance.dag_run)
            .filter(models.TaskInstance.dag_id.in_(list(self.dagbag.dag_ids)))
            .filter(models.TaskInstance.state.in_(old_states))
            .filter(
                or_(
                    
                    models.DagRun.state != State.RUNNING,
                    
                    models.DagRun.state.is_(None),
                )
            )
        )
        
        
        if self.using_sqlite or self.using_mysql:
            tis_to_change: List[TI] = with_row_locks(query, of=TI, **skip_locked(session=session)).all()
            for ti in tis_to_change:
                ti.set_state(new_state, session=session)
                tis_changed += 1
        else:
            subq = query.subquery()
            current_time = timezone.utcnow()
            ti_prop_update = {
                models.TaskInstance.state: new_state,
                models.TaskInstance.start_date: current_time,
            }
            
            if new_state in State.finished: