**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
([20, 11, 11, 11], 1, 1, "1d", "@once", "linear"),  
            ([20, 21, 23, 25], 1, 1, "1d", "30m", "no_structure"),  
            ([20, 21, 23, 25], 1, 1, "1d", "30m", "linear"),  
            ([20, 21, 23, 25], 1, 1, "1d", "30m", "binary_tree"),  
            ([20, 21, 23, 25], 1, 1, "1d", "30m", "star"),  
            ([20, 21, 23, 25], 1, 1, "1d", "30m", "grid"),  
            
            ([8, 8, 8, 8], 1, 5, "1d", "None", "no_structure"),  
            ([8, 8, 8, 8], 1, 5, "1d", "None", "linear"),  
            ([20, 11, 11, 11], 1, 5, "1d", "@once", "no_structure"),  
            ([21, 12, 12, 12], 1, 5, "1d", "@once", "linear"),  
            ([20, 21, 23, 25], 1, 5, "1d", "30m", "no_structure"),  
            ([21, 23, 26, 29], 1, 5, "1d", "30m", "linear"),  
            ([21, 23, 26, 29], 1, 5, "1d", "30m", "binary_tree"),  
            ([21, 23, 26, 29], 1, 5, "1d", "30m", "star"),  
            ([21, 23, 26, 29], 1, 5, "1d", "30m", "grid"),  
            
            ([8, 8, 8, 8], 10, 10, "1d", "None", "no_structure"),  
            ([8, 8, 8, 8], 10, 10, "1d", "None", "linear"),  
            ([83, 26, 26, 26], 10, 10, "1d", "@once", "no_structure"),  
            ([93, 39, 39, 39], 10, 10, "1d", "@once", "linear"),  
            ([83, 87, 87, 87], 10, 10, "1d", "30m", "no_structure"),  
            ([93, 113, 113, 113], 10, 10, "1d", "30m", "linear"),  
            ([93, 107, 107, 107], 10, 10, "1d", "30m", "binary_tree"),  
            ([93, 107, 107, 107], 10, 10, "1d", "30m", "star"),  
            ([93, 107, 107, 107], 10, 10, "1d", "30m", "grid"),  
        ]
    )
    @pytest.mark.quarantined
    def test_process_dags_queries_count(