We have seen this issue with the Kafka consumer, the new library that got introduced in 0.9
With this new client, the group management is done by kafka coordinator, which is one of the kafka broker.
We are using Kafka broker 0.10.2.1 and consumer client version is also 0.10.2.1
The issue that we have faced is that, after rebalancing, some of the partitions gets consumed by 2 instances within a consumer group, leading to duplication of the entire partition data. Both the instances continue to read until the next rebalancing, or the restart of those clients.
It looks like that a particular consumer goes on fetching the data from a partition, but the broker is not able to identify this "stale" consumer instance.
We have hit this twice in production. Please look at it the earliest.