title: Kafka delivering duplicate message
body: <p>We are using kafka(0.9.0.0) for orchestrating command messages between different micro services.  We are finding an intermittent issue where duplicate messages are getting delivered to a particular topic.  The logs that occur when this issue happens is given below.  Can some one help to understand this issue   </p>
  
  <pre><code>Wed, 21-Sep-2016 09:19:07 - WARNING Coordinator unknown during heartbeat -- will retry
  Wed, 21-Sep-2016 09:19:07 - WARNING Heartbeat failed; retrying
  Wed, 21-Sep-2016 09:19:07 - WARNING &lt;BrokerConnection host=AZSG-D-BOT-DEV4 port=9092&gt; timed out after 40000 ms. Closing connection.
  Wed, 21-Sep-2016 09:19:07 - ERROR Fetch to node 1 failed: RequestTimedOutError - 7 - This error is thrown if the request exceeds the user-specified time limit in the request.
  Wed, 21-Sep-2016 09:19:07 - INFO Marking the coordinator dead (node 1): None.
  Wed, 21-Sep-2016 09:19:07 - INFO Group coordinator for kafka-python-default-group is BrokerMetadata(nodeId=1, host=u'AZSG-D-BOT-DEV4', port=9092)
  Wed, 21-Sep-2016 09:19:07 - ERROR OffsetCommit failed for group kafka-python-default-group due to group error (UnknownMemberIdError - 25 - Returned from group requests (offset commits/fetches, heartbeats, etc) when the memberId is not in the current generation.), will rejoin
  Wed, 21-Sep-2016 09:19:07 - WARNING Offset commit failed: group membership out of date This is likely to cause duplicate message delivery.
  Wed, 21-Sep-2016 09:19:07 - ERROR LeaveGroup request failed: UnknownMemberIdError - 25 - Returned from group requests (offset commits/fetches, heartbeats, etc) when the memberId is not in the current generation.
  Wed, 21-Sep-2016 09:19:07 - INFO Marking the coordinator dead (node 1): None.
  Wed, 21-Sep-2016 09:19:07 - INFO Group coordinator for kafka-python-default-group is BrokerMetadata(nodeId=1, host=u'AZSG-D-BOT-DEV4', port=9092)
  Wed, 21-Sep-2016 09:19:07 - ERROR OffsetCommit failed for group kafka-python-default-group due to group error (UnknownMemberIdError - 25 - Returned from group requests (offset commits/fetches, heartbeats, etc) when the memberId is not in the current generation.), will rejoin
  Wed, 21-Sep-2016 09:19:07 - WARNING Offset commit failed: group membership out of date This is likely to cause duplicate message delivery.
  Wed, 21-Sep-2016 09:19:10 - INFO Joined group 'kafka-python-default-group' (generation 5) with member_id kafka-python-1.0.2-8585f310-cb4f-493a-a98d-12ec9810419b
  Wed, 21-Sep-2016 09:19:10 - INFO Updated partition assignment: [TopicPartition(topic=u'ilinaTestPlatformReq', partition=0)]
  </code></pre>