Prefix duplicate bug

[edmondchuc]

While working on this PR #134, I noticed tests would fail randomly due to multiple prefixes being generated for the same namespace.

For example, http://linked.data.gov.au/def/borehole-purpose generates def:borehole-purpose or df:borehole-purpose depending on the order of the prefix generation along with http://linked.data.gov.au/def/reg-status. This would cause tests to fail when testing certain endpoints such as /v/vocab/{concept_scheme_curie}/{concept_curie}/narrowers.

This is likely a bug in RDFLib's rdflib.Graph.compute_qname method where it doesn't check whether a namespace already exists before creating one, or something along those lines.

The fix is probably a change upstream in the RDFLib library to ensure that a namespace doesn't exist already before creating a new CURIE using compute_qname.