**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
try:
            sig_name = signal.Signals(signum).name  
        except Exception:  
            sig_name = str(signum)
        self.log.info("%s\n%s received, printing debug\n%s", "-" * 80, sig_name, "-" * 80)
        self.executor.debug_dump()
        self.log.info("-" * 80)
    def is_alive(self, grace_multiplier: Optional[float] = None) -> bool:
        """
        Is this SchedulerJob alive?
        We define alive as in a state of running and a heartbeat within the
        threshold defined in the ``scheduler_health_check_threshold`` config
        setting.
        ``grace_multiplier`` is accepted for compatibility with the parent class.
        :rtype: boolean
        """
        if grace_multiplier is not None:
            
            return super().is_alive(grace_multiplier=grace_multiplier)
        scheduler_health_check_threshold: int = conf.getint('scheduler', 'scheduler_health_check_threshold')
        return (
            self.state == State.RUNNING
            and (timezone.utcnow() - self.latest_heartbeat).total_seconds() < scheduler_health_check_threshold
        )
    @provide_session
    def _change_state_for_tis_without_dagrun(