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When running 'ant -f nbbuild/build.xml -Dnb.clusters.list=nb.cluster.platform build-javadoc', the target ignores that -Dnb.clusters.list=... part and wants to build complete javadoc (as obvious from the [echo] Repeat info). This is problem in case the whole source tree is not available, like in cases when user just has sources needed just for nb platform and wants to build it and the related javadoc.
Clusters are not considered for Javadoc builds. You need to set e.g. config.javadoc.all.