Duplicate key error

If you run the test-score-upload-from-sciunit.ipynb notebook in this repo, you will now get:
```
ERROR:scidash_api.client:SERVER -> SERVER ERROR: ['IntegrityError at /data/upload/data.json\nduplicate key value violates unique constraint "sciunitmodels_modelclass_url_0a12bafd_uniq"\nDETAIL: Key (url)=(https://github.com/scidash/neuronunit/blob/master/neuronunit/models/NeuroML2/LEMS_2007One.xml) already exists.
```

This was discussed in the #scidash channel on Slack and Dario has preliminary thoughts here: https://asuiconlab.slack.com/archives/CL7JKNG10/p1598466199014200

Run test-score-upload-from-sciunit.ipynb and reproduce the duplicate-key error at the data upload endpoint. Review the linked Slack discussion for Dario's preliminary thoughts and inspect how the existing URL uniqueness constraint is handled. The expected upload behavior and the final acceptance criteria still need clarification.

Written by the indexing model from the issue text.