**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/serialization/serialized_objects.py
v = SerializedDAG.deserialize_dag(v)
            elif k in {"retry_delay", "execution_timeout"}:
                v = cls._deserialize_timedelta(v)
            elif k in encoded_op["template_fields"]:
                pass
            elif k.endswith("_date"):
                v = cls._deserialize_datetime(v)
            elif k == "_operator_extra_links":
                op_predefined_extra_links = cls._deserialize_operator_extra_links(v)
                
                op_predefined_extra_links.update(op_extra_links_from_plugin)
                v = list(op_predefined_extra_links.values())
                k = "operator_extra_links"
            elif k == "deps":
                v = cls._deserialize_deps(v)
            elif k in cls._decorated_fields or k not in op.get_serialized_fields():
                v = cls._deserialize(v)
            
            setattr(op, k, v)
        for k in op.get_serialized_fields() - encoded_op.keys() - cls._CONSTRUCTOR_PARAMS.keys():
            setattr(op, k, None)
        
        for field in op.template_fields:
            if not hasattr(op, field):
                setattr(op, field, None)
        
        setattr(op, "_is_dummy", bool(encoded_op.get("_is_dummy", False)))
        return op
    @classmethod
    def _is_excluded(cls, var: Any, attrname: str, op: BaseOperator):