story 71751536. rdf:type triples are duplicated in output

Remove duplicates from RDFStream.

https://www.pivotaltracker.com/s/projects/684825/stories/71751536
It would be much better to do some kind of limited filtering. The extant filter() method could be reused for this. The whole purpose of RdfStream was originally precisely to avoid pulling all the triples into heap, because we saw severe performance degradations when that happened.