**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
.join(TI.dag_run)
            .filter(
                DagRun.run_type != DagRunType.BACKFILL_JOB,
                
                DagRun.state == State.RUNNING,
            )
            .options(load_only(TI.dag_id, TI.task_id, TI.execution_date))
        )
        
        tis_to_reset_or_adopt = with_row_locks(query, of=TI, **skip_locked(session=session)).all()
        to_reset = self.executor.try_adopt_task_instances(tis_to_reset_or_adopt)
        reset_tis_message = []
        for ti in to_reset:
            reset_tis_message.append(repr(ti))
            ti.state = State.NONE
            ti.queued_by_job_id = None
        for ti in set(tis_to_reset_or_adopt) - set(to_reset):
            ti.queued_by_job_id = self.id
        Stats.incr('scheduler.orphaned_tasks.cleared', len(to_reset))
        Stats.incr('scheduler.orphaned_tasks.adopted', len(tis_to_reset_or_adopt) - len(to_reset))
        if to_reset:
            task_instance_str = '\n\t'.join(reset_tis_message)
            self.log.info(
                "Reset the following %s orphaned TaskInstances:\n\t%s", len(to_reset), task_instance_str
            )
        
        
        session.flush()
        return len(to_reset)