story 71751536. rdf:type triples are duplicated in output

Remove duplicates from RDFStream.

https://www.pivotaltracker.com/s/projects/684825/stories/71751536
Why do we need to de-dupe? (And yes, @cbeer, putting these triples in an ArrayList or LinkedHashSet will pull them all into memory. Pretty much anything from the basic Java Collection API that impls Collection is an in-memory construct.)