**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/models/dagbag.py
path_hash = hashlib.sha1(filepath.encode('utf-8')).hexdigest()
        mod_name = f'unusual_prefix_{path_hash}_{org_mod_name}'
        if mod_name in sys.modules:
            del sys.modules[mod_name]
        timeout_msg = f"DagBag import timeout for {filepath} after {self.DAGBAG_IMPORT_TIMEOUT}s"
        with timeout(self.DAGBAG_IMPORT_TIMEOUT, error_message=timeout_msg):
            try:
                loader = importlib.machinery.SourceFileLoader(mod_name, filepath)
                spec = importlib.util.spec_from_loader(mod_name, loader)
                new_module = importlib.util.module_from_spec(spec)
                sys.modules[spec.name] = new_module
                loader.exec_module(new_module)
                return [new_module]
            except Exception as e:  
                self.log.exception("Failed to import: %s", filepath)
                if self.dagbag_import_error_tracebacks:
                    self.import_errors[filepath] = traceback.format_exc(
                        limit=-self.dagbag_import_error_traceback_depth
                    )
                else:
                    self.import_errors[filepath] = str(e)
        return []
    def _load_modules_from_zip(self, filepath, safe_mode):
        mods = []
        current_zip_file = zipfile.ZipFile(filepath)
        for zip_info in current_zip_file.infolist():
            head, _ = os.path.split(zip_info.filename)
            mod_name, ext = os.path.splitext(zip_info.filename)
            if ext not in [".py", ".pyc"]:
                continue