Stale read on existing ML classes in CreateClassifications

Sentry Issue: ARKINDEX-BACKEND-57

UniqueViolation: duplicate key value violates unique constraint "documents_mlclass_name_corpus_id_b4263e79_uniq"
DETAIL:  Key (name, corpus_id)=(o, d9ec9011-8491-4e76-b8e6-14556b8614e6) already exists.

  File "django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)

IntegrityError: duplicate key value violates unique constraint "documents_mlclass_name_corpus_id_b4263e79_uniq"
DETAIL:  Key (name, corpus_id)=(o, d9ec9011-8491-4e76-b8e6-14556b8614e6) already exists.

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  File "django/db/backends/utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "django/db/backends/utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "django/db/utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)