python - IntegrityError upon use of session.merge with sqlalchemy -

i use session.merge(obj1) insert , upsert rows. , occasionnally errors one:

(pymysql.err.integrityerror) (1062, u"duplicate entry 'q/ftmv4syu5qxencxvx6zghblsfysbo6kgv8sjuzdzww4j1zh3zfhtey/aqvoe5j' key 'primary'")  traceback   traceback (most recent call last):   file "/var/task/streaming_lambda.py", line 71, in lambda_handler     session.commit()   file "/var/task/sqlalchemy/orm/session.py", line 874, in commit     self.transaction.commit()   file "/var/task/sqlalchemy/orm/session.py", line 461, in commit     self._prepare_impl()   file "/var/task/sqlalchemy/orm/session.py", line 441, in _prepare_impl     self.session.flush()   file "/var/task/sqlalchemy/orm/session.py", line 2137, in flush     self._flush(objects)   file "/var/task/sqlalchemy/orm/session.py", line 2257, in _flush     transaction.rollback(_capture_exception=true)   file "/var/task/sqlalchemy/util/langhelpers.py", line 60, in __exit__     compat.reraise(exc_type, exc_value, exc_tb)   file "/var/task/sqlalchemy/orm/session.py", line 2221, in _flush     flush_context.execute()   file "/var/task/sqlalchemy/orm/unitofwork.py", line 389, in execute     rec.execute(self)   file "/var/task/sqlalchemy/orm/unitofwork.py", line 548, in execute     uow   file "/var/task/sqlalchemy/orm/persistence.py", line 181, in save_obj     mapper, table, insert)   file "/var/task/sqlalchemy/orm/persistence.py", line 799, in _emit_insert_statements     execute(statement, multiparams)   file "/var/task/sqlalchemy/engine/base.py", line 945, in execute     return meth(self, multiparams, params)   file "/var/task/sqlalchemy/sql/elements.py", line 263, in _execute_on_connection     return connection._execute_clauseelement(self, multiparams, params)   file "/var/task/sqlalchemy/engine/base.py", line 1053, in _execute_clauseelement     compiled_sql, distilled_params   file "/var/task/sqlalchemy/engine/base.py", line 1189, in _execute_context     context)   file "/var/task/sqlalchemy/engine/base.py", line 1393, in _handle_dbapi_exception     exc_info   file "/var/task/sqlalchemy/util/compat.py", line 202, in raise_from_cause     reraise(type(exception), exception, tb=exc_tb, cause=cause)   file "/var/task/sqlalchemy/engine/base.py", line 1182, in _execute_context     context)   file "/var/task/sqlalchemy/engine/default.py", line 469, in do_execute     cursor.execute(statement, parameters)   file "/var/task/pymysql/cursors.py", line 166, in execute     result = self._query(query)   file "/var/task/pymysql/cursors.py", line 322, in _query     conn.query(q)   file "/var/task/pymysql/connections.py", line 835, in query     self._affected_rows = self._read_query_result(unbuffered=unbuffered)   file "/var/task/pymysql/connections.py", line 1019, in _read_query_result     result.read()   file "/var/task/pymysql/connections.py", line 1302, in read     first_packet = self.connection._read_packet()   file "/var/task/pymysql/connections.py", line 981, in _read_packet     packet.check_error()   file "/var/task/pymysql/connections.py", line 393, in check_error     err.raise_mysql_exception(self._data)   file "/var/task/pymysql/err.py", line 107, in raise_mysql_exception     raise errorclass(errno, errval) integrityerror: (pymysql.err.integrityerror) (1062, u"duplicate entry 'q/ftmv4syu5qxencxvx6zghblsfysbo6kgv8sjuzdzww4j1zh3zfhtey/aqvoe5j' key 'primary'") [sql: u'insert user_table (_id, collection_name, platform, created_at, updated_at, deleted, account_complete, population_current, state, stars, language, age_range, private_account, user_credit, new_user, anonymous) values (%(_id)s, %(collection_name)s, %(platform)s, %(created_at)s, %(updated_at)s, %(deleted)s, %(account_complete)s, %(population_current)s, %(state)s, %(stars)s, %(language)s, %(age_range)s, %(private_account)s, %(user_credit)s, %(new_user)s, %(anonymous)s)'] [parameters: {'user_credit': 0, 'age_range': none, 'anonymous': 0, 'language': none, 'new_user': 1, 'collection_name': none, 'created_at': datetime.datetime(2000, 1, 1, 0, 0), 'deleted': 0, 'account_complete': none, 'updated_at': none, 'platform': 'undefined', 'state': none, 'stars': none, '_id': 'q/ftmv4syu5qxencxvx6zghblsfysbo6kgv8sjuzdzww4j1zh3zfhtey/aqvoe5jms3lkmkns0lkrc+uzeifdhg32wqwg6efm+h4ofptf1kzskip54yj243av+lqfsrqojd5seocn178sodeamfeeprhzy3s9nw6ro1e64zpxoy=', 'private_account': 0, 'population_current': 'undefined'}] 

i'm using sqlalchemy 0.9.1 , pymysql 0.7.9 communicate in mysql rds mysql database. how can session.merge() generate duplicate entry error? isn't purpose?