Edges can be reified multiple times if the client calls addFact when the fact already exists

[balhoff]

This seems to be happening in cases like this: https://github.com/geneontology/noctua-models/blob/master/models/68d5ebd600003246.ttl#L903C1-L919C98

Candidate fix in #586, but I'm not entirely sure we want to perform a different operation than was explicitly requested by the client.

[vanaukenk]

Thanks for the PR @balhoff

I'll attempt some testing on noctua-dev so we can see how things go.