**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
dag2.sync_to_db(session=session)
        ti1 = TaskInstance(task1, DEFAULT_DATE)
        ti1.state = State.QUEUED
        session.merge(ti1)
        session.commit()
        executor.event_buffer[ti1.key] = State.FAILED, None
        scheduler._process_executor_events(session=session)
        ti1.refresh_from_db()
        assert ti1.state == State.QUEUED
        mock_task_callback.assert_called_once_with(
            full_filepath='/test_path1/',
            simple_task_instance=mock.ANY,
            msg='Executor reports task instance '
            '<TaskInstance: test_process_executor_events.dummy_task 2016-01-01 00:00:00+00:00 [queued]> '
            'finished (failed) although the task says its queued. (Info: None) '
            'Was the task killed externally?',
        )
        scheduler.processor_agent.send_callback_to_execute.assert_called_once_with(task_callback)
        scheduler.processor_agent.reset_mock()
        
        ti1.state = State.SUCCESS
        session.merge(ti1)
        session.commit()
        executor.event_buffer[ti1.key] = State.SUCCESS, None
        scheduler._process_executor_events(session=session)
        ti1.refresh_from_db()
        assert ti1.state == State.SUCCESS
        scheduler.processor_agent.send_callback_to_execute.assert_not_called()
        mock_stats_incr.assert_called_once_with('scheduler.tasks.killed_externally')
    def test_process_executor_events_uses_inmemory_try_number(self):