No callers of MultiIndexBlockImpl::insertAllDocumentsInCollection(std::set<RecordId>* dupsOut = nullptr), except for an index unit test, pass in a function parameter. They all let the function parameter default to nullptr.

I suspect that this field was outdated with the introduction of the 'unique' index build setting. A build will handle the issue of duplicates internally, succeeding or failing depending on settings that were passed into the index build request.