**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
"""Launch the process and start processing the DAG."""
        start_method = self._get_multiprocessing_start_method()
        context = multiprocessing.get_context(start_method)
        _parent_channel, _child_channel = context.Pipe(duplex=False)
        process = context.Process(
            target=type(self)._run_file_processor,
            args=(
                _child_channel,
                _parent_channel,
                self.file_path,
                self._pickle_dags,
                self._dag_ids,
                f"DagFileProcessor{self._instance_id}",
                self._callback_requests,
            ),
            name=f"DagFileProcessor{self._instance_id}-Process",
        )
        self._process = process
        self._start_time = timezone.utcnow()
        process.start()
        
        
        _child_channel.close()
        del _child_channel
        
        
        self._parent_channel = _parent_channel
    def kill(self) -> None: