**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/models/dagbag.py
if self.read_dags_from_db:
            return
        self.log.info("Filling up the DagBag from %s", dag_folder)
        dag_folder = dag_folder or self.dag_folder
        
        stats = []
        dag_folder = correct_maybe_zipped(dag_folder)
        for filepath in list_py_file_paths(
            dag_folder,
            safe_mode=safe_mode,
            include_examples=include_examples,
            include_smart_sensor=include_smart_sensor,
        ):
            try:
                file_parse_start_dttm = timezone.utcnow()
                found_dags = self.process_file(filepath, only_if_updated=only_if_updated, safe_mode=safe_mode)
                file_parse_end_dttm = timezone.utcnow()
                stats.append(
                    FileLoadStat(
                        file=filepath.replace(settings.DAGS_FOLDER, ''),
                        duration=file_parse_end_dttm - file_parse_start_dttm,
                        dag_num=len(found_dags),
                        task_num=sum([len(dag.tasks) for dag in found_dags]),
                        dags=str([dag.dag_id for dag in found_dags]),
                    )
                )
            except Exception as e:  
                self.log.exception(e)
        self.dagbag_stats = sorted(stats, key=lambda x: x.duration, reverse=True)
    def collect_dags_from_db(self):