**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
for attempt in tenacity.Retrying(
            retry=tenacity.retry_if_exception_type(exception_types=OperationalError),
            wait=tenacity.wait_random_exponential(multiplier=0.5, max=5),
            stop=tenacity.stop_after_attempt(settings.MAX_DB_RETRIES),
            before_sleep=tenacity.before_sleep_log(self.log, logging.DEBUG),
            reraise=True,
        ):
            with attempt:
                serialize_errors = []
                self.log.debug(
                    "Running dagbag.sync_to_db with retries. Try %d of %d",
                    attempt.retry_state.attempt_number,
                    settings.MAX_DB_RETRIES,
                )
                self.log.debug("Calling the DAG.bulk_sync_to_db method")
                try:
                    
                    for dag in self.dags.values():
                        serialize_errors.extend(_serialze_dag_capturing_errors(dag, session))
                    DAG.bulk_write_to_db(self.dags.values(), session=session)
                except OperationalError:
                    session.rollback()
                    raise
                
                
                self.import_errors.update(dict(serialize_errors))