story 71751536. rdf:type triples are duplicated in output

Remove duplicates from RDFStream.

https://www.pivotaltracker.com/s/projects/684825/stories/71751536
I think we should de-dupe the rdf:type statements -- it looks sloppy to repeat them.

Ideally, we would be able to avoid adding the duplicate types in the first place.  But if that doesn't work, then a better pattern would be an Iterator implementation that wraps the triples and suppresses duplicate rdf:type triples.  We would only need the rdf:type triples in memory, which would avoid the typical cases where this would be a problem (e.g. large numbers of children, etc.).