**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.

tests/jobs/test_scheduler_job.py
else:
                assert start_date is None
                assert end_date is None
                assert duration is None
@pytest.mark.usefixtures("disable_load_example")
class TestSchedulerJob(unittest.TestCase):
    def setUp(self):
        clear_db_runs()
        clear_db_pools()
        clear_db_dags()
        clear_db_sla_miss()
        clear_db_errors()
        
        
        self.null_exec = MockExecutor()
        self.patcher = patch('airflow.utils.dag_processing.SerializedDagModel.remove_deleted_dags')
        
        self.patcher_dag_code = patch.object(settings, "STORE_DAG_CODE", False)
        self.patcher.start()
        self.patcher_dag_code.start()
    def tearDown(self):
        self.patcher.stop()
        self.patcher_dag_code.stop()
    @classmethod
    def setUpClass(cls):
        
        non_serialized_dagbag = DagBag(read_dags_from_db=False, include_examples=False)
        non_serialized_dagbag.sync_to_db()
        cls.dagbag = DagBag(read_dags_from_db=True)
    def test_is_alive(self):