**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
self.dags_hash: Dict[str, str] = {}
        self.dagbag_import_error_tracebacks = conf.getboolean('core', 'dagbag_import_error_tracebacks')
        self.dagbag_import_error_traceback_depth = conf.getint('core', 'dagbag_import_error_traceback_depth')
        self.collect_dags(
            dag_folder=dag_folder,
            include_examples=include_examples,
            include_smart_sensor=include_smart_sensor,
            safe_mode=safe_mode,
        )
    def size(self) -> int:
        """:return: the amount of dags contained in this dagbag"""
        return len(self.dags)
    @property
    def store_serialized_dags(self) -> bool:
        """Whether or not to read dags from DB"""
        warnings.warn(
            "The store_serialized_dags property has been deprecated. Use read_dags_from_db instead.",
            DeprecationWarning,
            stacklevel=2,
        )
        return self.read_dags_from_db
    @property
    def dag_ids(self) -> List[str]:
        """
        :return: a list of DAG IDs in this bag
        :rtype: List[unicode]
        """
        return list(self.dags.keys())
    @provide_session
    def get_dag(self, dag_id, session: Session = None):