**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_prop_update.update(
                    {
                        models.TaskInstance.end_date: current_time,
                        models.TaskInstance.duration: 0,
                    }
                )
            tis_changed = (
                session.query(models.TaskInstance)
                .filter(
                    models.TaskInstance.dag_id == subq.c.dag_id,
                    models.TaskInstance.task_id == subq.c.task_id,
                    models.TaskInstance.execution_date == subq.c.execution_date,
                )
                .update(ti_prop_update, synchronize_session=False)
            )
        if tis_changed > 0:
            session.flush()
            self.log.warning(
                "Set %s task instances to state=%s as their associated DagRun was not in RUNNING state",
                tis_changed,
                new_state,
            )
            Stats.gauge('scheduler.tasks.without_dagrun', tis_changed)
    @provide_session
    def __get_concurrency_maps(
        self, states: List[str], session: Session = None
    ) -> Tuple[DefaultDict[str, int], DefaultDict[Tuple[str, str], int]]:
        """
        Get the concurrency maps.
        :param states: List of states to query for