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https://issues.apache.org/jira/browse/KAFKA-4609?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16959455#comment-16959455
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Matthias J. Sax commented on KAFKA-4609:
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As you can see, the ticket is open. It was never addressed and thus it's not a
surprise that you may still hit it in never versions.
> KTable/KTable join followed by groupBy and aggregate/count can result in
> duplicated results
> -------------------------------------------------------------------------------------------
>
> Key: KAFKA-4609
> URL: https://issues.apache.org/jira/browse/KAFKA-4609
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Affects Versions: 0.10.1.1, 0.10.2.0
> Reporter: Damian Guy
> Priority: Major
> Labels: architecture
>
> When caching is enabled, KTable/KTable joins can result in duplicate values
> being emitted. This will occur if there were updates to the same key in both
> tables. Each table is flushed independently, and each table will trigger the
> join, so you get two results for the same key.
> If we subsequently perform a groupBy and then aggregate operation we will now
> process these duplicates resulting in incorrect aggregated values. For
> example count will be double the value it should be.
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