When using zmq, every event is processed two times

2016-05-24 12:53:33.827 22207 INFO dragonflow.db.api_nb [-] Pushing Update to Queue: Action:delete, Table:lport, Key:eff0afad-f3e5-4cc5-84d5-b2d86c20e0dd Value:None Topic:D
2016-05-24 12:53:33.828 22207 DEBUG dragonflow.db.api_nb [-] Event update: Action:delete, Table:lport, Key:eff0afad-f3e5-4cc5-84d5-b2d86c20e0dd Value:None Topic:D _read_db_changes_from_queue /opt/stack/new/dragonflow/dragonflow/db/api_nb.py:163
2016-05-24 12:53:33.828 22207 INFO dragonflow.db.api_nb [-] Pushing Update to Queue: Action:delete, Table:lport, Key:eff0afad-f3e5-4cc5-84d5-b2d86c20e0dd Value:None Topic:D
2016-05-24 12:53:33.829 22207 DEBUG dragonflow.db.api_nb [-] Event update: Action:delete, Table:lport, Key:eff0afad-f3e5-4cc5-84d5-b2d86c20e0dd Value:None Topic:D _read_db_changes_from_queue /opt/stack/new/dragonflow/dragonflow/db/api_nb.py:163
2016-05-24 12:53:34.090 22207 INFO dragonflow.db.api_nb [-] Pushing Update to Queue: Action:delete, Table:lswitch, Key:b1bf678e-5fd2-460b-87aa-51106560c32a Value:None Topic:D
2016-05-24 12:53:34.090 22207 DEBUG dragonflow.db.api_nb [-] Event update: Action:delete, Table:lswitch, Key:b1bf678e-5fd2-460b-87aa-51106560c32a Value:None Topic:D _read_db_changes_from_queue /opt/stack/new/dragonflow/dragonflow/db/api_nb.py:163
2016-05-24 12:53:34.091 22207 INFO dragonflow.controller.df_local_controller [-] Removing Logical Switch = {u'subnets': [], u'name': u'mynetwork1', u'router_external': False, u'topic': u'dd8c7aa1901a469b8fca9f31deb3a3e7', u'version': 4, u'id': u'b1bf678e-5fd2-460b-87aa-51106560c32a'}
2016-05-24 12:53:34.093 22207 INFO dragonflow.db.api_nb [-] Pushing Update to Queue: Action:delete, Table:lswitch, Key:b1bf678e-5fd2-460b-87aa-51106560c32a Value:None Topic:D
2016-05-24 12:53:34.093 22207 DEBUG dragonflow.db.api_nb [-] Event update: Action:delete, Table:lswitch, Key:b1bf678e-5fd2-460b-87aa-51106560c32a Value:None Topic:D _read_db_changes_from_queue /opt/stack/new/dragonflow/dragonflow/db/api_nb.py:163
2016-05-24 12:53:34.093 22207 INFO dragonflow.controller.df_local_controller [-] Removing Logical Switch = None
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb [-] 'NoneType' object has no attribute 'get_id'
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb Traceback (most recent call last):
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb File "/opt/stack/new/dragonflow/dragonflow/db/api_nb.py", line 175, in _read_db_changes_from_queue
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb value)
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb File "/opt/stack/new/dragonflow/dragonflow/db/api_nb.py", line 231, in apply_db_change
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb self.controller.logical_switch_deleted(lswitch_id)
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb File "/opt/stack/new/dragonflow/dragonflow/controller/df_local_controller.py", line 187, in logical_switch_deleted
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb self.open_flow_app.notify_remove_logical_switch(lswitch)
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb File "/opt/stack/new/dragonflow/dragonflow/controller/ryu_base_app.py", line 84, in notify_remove_logical_switch
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb self.dispatcher.dispatch('remove_logical_switch', lswitch=lswitch)
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb File "/opt/stack/new/dragonflow/dragonflow/controller/dispatcher.py", line 43, in dispatch
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb handler(*args, **kwargs)
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb File "/opt/stack/new/dragonflow/dragonflow/controller/l2_app.py", line 333, in remove_logical_switch
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb lswitch.get_id(),
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb AttributeError: 'NoneType' object has no attribute 'get_id'
2016-05-24 12:53:34.093 22207 ERROR dragonflow.db.api_nb