**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
)
        for task_instance in tis_to_set_to_scheduled:
            self.executor.queued_tasks.pop(task_instance.key)
        task_instance_str = "\n\t".join(repr(x) for x in tis_to_set_to_scheduled)
        self.log.info("Set the following tasks to scheduled state:\n\t%s", task_instance_str)
    @provide_session
    def _process_executor_events(self, session: Session = None) -> int:
        """Respond to executor events."""
        if not self.processor_agent:
            raise ValueError("Processor agent is not started.")
        ti_primary_key_to_try_number_map: Dict[Tuple[str, str, datetime.datetime], int] = {}
        event_buffer = self.executor.get_event_buffer()
        tis_with_right_state: List[TaskInstanceKey] = []
        
        for ti_key, value in event_buffer.items():
            state: str
            state, _ = value
            
            ti_primary_key_to_try_number_map[ti_key.primary] = ti_key.try_number
            self.log.info(
                "Executor reports execution of %s.%s execution_date=%s "
                "exited with status %s for try_number %s",
                ti_key.dag_id,
                ti_key.task_id,
                ti_key.execution_date,
                state,
                ti_key.try_number,
            )
            if state in (State.FAILED, State.SUCCESS, State.QUEUED):
                tis_with_right_state.append(ti_key)