Consider one job that is run on different slaves at different times. Each slave reports a Mercurial SCM changelog relative to its own working copy, which results in one Mercurial changeset showing up in multiple builds of the same job.

Proposed solution: The master node should keep a collection (a HashSet) with all unique changeset IDs. Every time a slave sends a new ID, the master must verify if it's really new. If the ID is found in the HashSet, the master can simply skip it or refrain from printing it.