**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
clear_db_errors,
    clear_db_jobs,
    clear_db_pools,
    clear_db_runs,
    clear_db_serialized_dags,
    clear_db_sla_miss,
    set_default_pool_slots,
)
from tests.test_utils.mock_executor import MockExecutor
ROOT_FOLDER = os.path.realpath(
    os.path.join(os.path.dirname(os.path.realpath(__file__)), os.pardir, os.pardir)
)
PERF_DAGS_FOLDER = os.path.join(ROOT_FOLDER, "tests", "test_utils", "perf", "dags")
ELASTIC_DAG_FILE = os.path.join(PERF_DAGS_FOLDER, "elastic_dag.py")
TEST_DAG_FOLDER = os.environ['AIRFLOW__CORE__DAGS_FOLDER']
DEFAULT_DATE = timezone.datetime(2016, 1, 1)
TRY_NUMBER = 1
PARSEABLE_DAG_FILE_CONTENTS = '"airflow DAG"'
UNPARSEABLE_DAG_FILE_CONTENTS = 'airflow DAG'
INVALID_DAG_WITH_DEPTH_FILE_CONTENTS = "def something():\n    return airflow_DAG\nsomething()"
TEMP_DAG_FILENAME = "temp_dag.py"
@pytest.fixture(scope="class")
def disable_load_example():
    with conf_vars({('core', 'load_examples'): 'false'}):
        with env_vars({('core', 'load_examples'): 'false'}):
            yield
@pytest.mark.usefixtures("disable_load_example")
class TestDagFileProcessor(unittest.TestCase):
    @staticmethod
    def clean_db():