public static IPipeline WithProcessDocumentsOnce(this IPipeline pipeline, bool processDocumentsOnce = true)

when set, the pipeline looks for the first occurrence of a given Wyam.Common.Documents.IDocument.Source and then caches all final result documents that have the same source. On subsequent executions, if a document with a previously seen Wyam.Common.Documents.IDocument.Source is found and it has the same content, that document is removed from the module output and therefore won't get passed to the next module. At the end of the pipeline, all the documents from the first pass that have the same source as the removed one are added back to the result set (so later pipelines can still access them in the documents collection if needed).