**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**: tencent cloud
- **OS** (e.g. from /etc/os-release): centos7
- **Kernel** (e.g. `uname -a`): 3.10
- **Install tools**:
- **Others**: Server version: 8.0.22 MySQL Community Server - GPL

**What happened**:

Scheduler died when I try to modify a dag's schedule_interval from "None" to "* */1 * * *"(I edited the dag file in the dag folder and saved it). A few minutes later I tried to start the scheduler again and it began to run.

And the logs are as follows:
```
{2021-01-15 09:06:22,636} {{scheduler_job.py:1293}} ERROR - Exception when executing SchedulerJob._run_scheduler_loop
Traceback (most recent call last):
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1276, in _execute_context
    self.dialect.do_execute(
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/engine/default.py", line 609, in do_execute
    cursor.execute(statement, parameters)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/MySQLdb/cursors.py", line 209, in execute
    res = self._query(query)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/MySQLdb/cursors.py", line 315, in _query
    db.query(q)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/MySQLdb/connections.py", line 239, in query
    _mysql.connection.query(self, query)
MySQLdb._exceptions.IntegrityError: (1062, "Duplicate entry 'huge_demo13499411352-2021-01-15 01:04:00.000000' for key 'dag_run.dag_id'")

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1275, in _execute
    self._run_scheduler_loop()
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1377, in _run_scheduler_loop
    num_queued_tis = self._do_scheduling(session)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1474, in _do_scheduling
    self._create_dag_runs(query.all(), session)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1561, in _create_dag_runs
    dag.create_dagrun(
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/airflow/utils/session.py", line 62, in wrapper
    return func(*args, **kwargs)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/airflow/models/dag.py", line 1807, in create_dagrun
    session.flush()
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/orm/session.py", line 2540, in flush
    self._flush(objects)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/orm/session.py", line 2682, in _flush
    transaction.rollback(_capture_exception=True)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/util/langhelpers.py", line 68, in __exit__
    compat.raise_(
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/orm/session.py", line 2642, in _flush
    flush_context.execute()
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/orm/unitofwork.py", line 422, in execute
    rec.execute(self)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/orm/unitofwork.py", line 586, in execute
    persistence.save_obj(
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/orm/persistence.py", line 239, in save_obj
    _emit_insert_statements(
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/orm/persistence.py", line 1135, in _emit_insert_statements
    result = cached_connections[connection].execute(
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1011, in execute
    return meth(self, multiparams, params)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/sql/elements.py", line 298, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1124, in _execute_clauseelement
    ret = self._execute_context(
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1316, in _execute_context
    self._handle_dbapi_exception(
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1510, in _handle_dbapi_exception
    util.raise_(
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 1276, in _execute_context
    self.dialect.do_execute(
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/sqlalchemy/engine/default.py", line 609, in do_execute
    cursor.execute(statement, parameters)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/MySQLdb/cursors.py", line 209, in execute
    res = self._query(query)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/MySQLdb/cursors.py", line 315, in _query
    db.query(q)
  File "/home/app/.pyenv/versions/3.8.1/envs/airflow-py381/lib/python3.8/site-packages/MySQLdb/connections.py", line 239, in query
    _mysql.connection.query(self, query)
sqlalchemy.exc.IntegrityError: (MySQLdb._exceptions.IntegrityError) (1062, "Duplicate entry 'huge_demo13499411352-2021-01-15 01:04:00.000000' for key 'dag_run.dag_id'")
[SQL: INSERT INTO dag_run (dag_id, execution_date, start_date, end_date, state, run_id, creating_job_id, external_trigger, run_type, conf, last_scheduling_decision, dag_hash) VALUES (%s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s)]
[parameters: ('huge_demo13499411352', datetime.datetime(2021, 1, 15, 1, 4), datetime.datetime(2021, 1, 15, 1, 6, 22, 629433), None, 'running', 'scheduled__2021-01-15T01:04:00+00:00', 71466, 0, <DagRunType.SCHEDULED: 'scheduled'>, b'\x80\x05}\x94.', None, '60078c379cdeecb9bc8844eed5aa9745')]
(Background on this error at: http://sqlalche.me/e/13/gkpj)
{2021-01-15 09:06:23,648} {{process_utils.py:95}} INFO - Sending Signals.SIGTERM to GPID 66351
{2021-01-15 09:06:23,781} {{process_utils.py:61}} INFO - Process psutil.Process(pid=66351, status='terminated') (66351) terminated with exit code 0
{2021-01-15 09:06:23,781} {{scheduler_job.py:1296}} INFO - Exited execute loop
```

**What you expected to happen**:

Schdeduler should not die.

**How to reproduce it**:

I don't know how to reproduce it

**Anything else we need to know**:

No

airflow/jobs/scheduler_job.py
#
#
#
#
import datetime
import itertools
import logging
import multiprocessing
import os
import sched
import signal
import sys
import threading
import time
from collections import defaultdict
from contextlib import redirect_stderr, redirect_stdout, suppress
from datetime import timedelta
from multiprocessing.connection import Connection as MultiprocessingConnection
from typing import Any, Callable, DefaultDict, Dict, Iterable, List, Optional, Set, Tuple
from setproctitle import setproctitle
from sqlalchemy import and_, func, not_, or_
from sqlalchemy.exc import OperationalError
from sqlalchemy.orm import load_only, selectinload
from sqlalchemy.orm.session import Session, make_transient
from airflow import models, settings