**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
"""
        If there are tasks left over in the executor,
        we set them back to SCHEDULED to avoid creating hanging tasks.
        :param session: session for ORM operations
        """
        if not self.executor.queued_tasks:
            return
        filter_for_ti_state_change = [
            and_(
                TI.dag_id == dag_id,
                TI.task_id == task_id,
                TI.execution_date == execution_date,
                
                
                TI._try_number == try_number - 1,  
                TI.state == State.QUEUED,
            )
            for dag_id, task_id, execution_date, try_number in self.executor.queued_tasks.keys()
        ]
        ti_query = session.query(TI).filter(or_(*filter_for_ti_state_change))
        tis_to_set_to_scheduled: List[TI] = with_row_locks(ti_query).all()
        if not tis_to_set_to_scheduled:
            return
        
        filter_for_tis = TI.filter_for_tis(tis_to_set_to_scheduled)
        session.query(TI).filter(filter_for_tis).update(
            {TI.state: State.SCHEDULED, TI.queued_dttm: None}, synchronize_session=False