**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,
        dag_folder: Optional[str] = None,
        include_examples: bool = conf.getboolean('core', 'LOAD_EXAMPLES'),
        include_smart_sensor: bool = conf.getboolean('smart_sensor', 'USE_SMART_SENSOR'),
        safe_mode: bool = conf.getboolean('core', 'DAG_DISCOVERY_SAFE_MODE'),
        read_dags_from_db: bool = False,
        store_serialized_dags: Optional[bool] = None,
    ):
        
        from airflow.models.dag import DAG
        super().__init__()
        if store_serialized_dags:
            warnings.warn(
                "The store_serialized_dags parameter has been deprecated. "
                "You should pass the read_dags_from_db parameter.",
                DeprecationWarning,
                stacklevel=2,
            )
            read_dags_from_db = store_serialized_dags
        dag_folder = dag_folder or settings.DAGS_FOLDER
        self.dag_folder = dag_folder
        self.dags: Dict[str, DAG] = {}
        
        self.file_last_changed: Dict[str, datetime] = {}
        self.import_errors: Dict[str, str] = {}
        self.has_logged = False
        self.read_dags_from_db = read_dags_from_db
        
        self.dags_last_fetched: Dict[str, datetime] = {}