**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
TI.dag_id == dag.dag_id,
                TI.task_id == qry.c.task_id,
                TI.execution_date == qry.c.max_ti,
            )
            .all()
        )
        ts = timezone.utcnow()
        for ti in max_tis:
            task = dag.get_task(ti.task_id)
            if not isinstance(task.sla, timedelta):
                continue
            dttm = dag.following_schedule(ti.execution_date)
            while dttm < timezone.utcnow():
                following_schedule = dag.following_schedule(dttm)
                if following_schedule + task.sla < timezone.utcnow():
                    session.merge(
                        SlaMiss(task_id=ti.task_id, dag_id=ti.dag_id, execution_date=dttm, timestamp=ts)
                    )
                dttm = dag.following_schedule(dttm)
        session.commit()
        
        slas: List[SlaMiss] = (
            session.query(SlaMiss)
            .filter(SlaMiss.notification_sent == False, SlaMiss.dag_id == dag.dag_id)  
            .all()
        )
        
        if slas:  
            sla_dates: List[datetime.datetime] = [sla.execution_date for sla in slas]
            fetched_tis: List[TI] = (