**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/jobs/scheduler_job.py
dag = dagbag.dags[request.dag_id]
        dag_run = dag.get_dagrun(execution_date=request.execution_date, session=session)
        dag.handle_callback(
            dagrun=dag_run, success=not request.is_failure_callback, reason=request.msg, session=session
        )
    def _execute_task_callbacks(self, dagbag: DagBag, request: TaskCallbackRequest):
        simple_ti = request.simple_task_instance
        if simple_ti.dag_id in dagbag.dags:
            dag = dagbag.dags[simple_ti.dag_id]
            if simple_ti.task_id in dag.task_ids:
                task = dag.get_task(simple_ti.task_id)
                ti = TI(task, simple_ti.execution_date)
                
                ti.start_date = simple_ti.start_date
                ti.end_date = simple_ti.end_date
                ti.try_number = simple_ti.try_number
                ti.state = simple_ti.state
                ti.test_mode = self.UNIT_TEST_MODE
                if request.is_failure_callback:
                    ti.handle_failure_with_callback(error=request.msg, test_mode=ti.test_mode)
                    self.log.info('Executed failure callback for %s in state %s', ti, ti.state)
    @provide_session
    def process_file(