Duplicate LOOM mappings returned from /classes/:cls/mappings endpoint

[jvendetti]

To reproduce: issue a REST call to get all the mappings for the "Tuberculosis" class in the ORDO ontology:

https://data.bioontology.org/ontologies/ORDO/classes/http%3A%2F%2Fwww.orpha.net%2FORDO%2FOrphanet_3389/mappings?display_context=false&display_links=false

In the result set, notice that there's a duplicate set of LOOM mappings that map from Tuberculosis in ORDO to Tuberculosis in the private SNO_COPY ontology:

[graybeal]

See also https://github.com/ncbo/ontologies_linked_data/issues/117.