airflow variable error

[2019-12-12 12:08:42,108] {{dagbag.py:207}} ERROR - Failed to import: /usr/local/airflow/dags/youtube_translation_encyclopedia.py
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1246, in _execute_context
    cursor, statement, parameters, context
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/default.py", line 581, in do_execute
    cursor.execute(statement, parameters)
sqlite3.IntegrityError: UNIQUE constraint failed: variable.key

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

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/airflow/models/dagbag.py", line 204, in process_file
    m = imp.load_source(mod_name, filepath)
  File "/usr/local/lib/python3.7/imp.py", line 171, in load_source
    module = _load(spec)
  File "<frozen importlib._bootstrap>", line 696, in _load
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/usr/local/airflow/dags/youtube_translation_encyclopedia.py", line 39, in <module>
    Variable.set("youtube_translation_encyclopedia_BATCH_START", 0)
  File "/usr/local/lib/python3.7/site-packages/airflow/utils/db.py", line 74, in wrapper
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/models/variable.py", line 142, in set
    session.flush()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 2470, in flush
    self._flush(objects)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 2608, in _flush
    transaction.rollback(_capture_exception=True)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 68, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 153, in reraise
    raise value
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 2568, in _flush
    flush_context.execute()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/unitofwork.py", line 422, in execute
    rec.execute(self)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/unitofwork.py", line 589, in execute
    uow,
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/persistence.py", line 245, in save_obj
    insert,
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/persistence.py", line 1137, in _emit_insert_statements
    statement, params
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 982, in execute
    return meth(self, multiparams, params)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/sql/elements.py", line 287, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1101, in _execute_clauseelement
    distilled_params,
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1250, in _execute_context
    e, statement, parameters, cursor, context
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1476, in _handle_dbapi_exception
    util.raise_from_cause(sqlalchemy_exception, exc_info)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 398, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 152, in reraise
    raise value.with_traceback(tb)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1246, in _execute_context
    cursor, statement, parameters, context
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/default.py", line 581, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) UNIQUE constraint failed: variable.key
[SQL: INSERT INTO variable ("key", val, is_encrypted) VALUES (?, ?, ?)]
[parameters: ('youtube_translation_encyclopedia_BATCH_START', 'gAAAAABd8i3K7rF0RC2LP6qTcbCo1oQ9093gDEUAgmjw1tJBCxgT3c_Qg1KcCkowy5EK4EQEw3rcNQp5usxWHqnRMrxHuzfC_Q==', 1)]
(Background on this error at: http://sqlalche.me/e/gkpj)