**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
"""
        Gets the DAG out of the dictionary, and refreshes it if expired
        :param dag_id: DAG Id
        :type dag_id: str
        """
        
        from airflow.models.dag import DagModel
        if self.read_dags_from_db:
            
            from airflow.models.serialized_dag import SerializedDagModel
            if dag_id not in self.dags:
                
                self._add_dag_from_db(dag_id=dag_id, session=session)
                return self.dags.get(dag_id)
            
            
            
            
            min_serialized_dag_fetch_secs = timedelta(seconds=settings.MIN_SERIALIZED_DAG_FETCH_INTERVAL)
            if (
                dag_id in self.dags_last_fetched
                and timezone.utcnow() > self.dags_last_fetched[dag_id] + min_serialized_dag_fetch_secs
            ):
                sd_last_updated_datetime = SerializedDagModel.get_last_updated_datetime(
                    dag_id=dag_id,
                    session=session,