From https://twitter.com/pdxleif/status/216415258829520896 Leif points out
One surprise with Swish.RDF.RDFGraph.toRDFGraph - not a set of triples? Should do (toRDFGraph . nub) to be safe?
which is true. I plan to convert NSGraph to use a set rather than a list but first I want to set up some benchmarks.
Note that the presence of repeated triples may not cause problems (e.g. graphs will still be considered equal) but you can see surprising results; e.g. when using formatGraphAsText or perhaps in some of the query routines (I have not done a thorough audit/check).