Investigate duplicate entries when using hubData::load_model_metadata

[sbidari]

No description provided.

[dylanhmorris]

It appears to return duplicate rows for different versions of the metadata when metadata that did not initially include designated_github_users has been updated to include it. I note that other updated metadata (e.g. for Covidhub-baseline, which did not initially have contributors listed) does not seem to create duplicate rows. My guess is that this has to do with adding a field versus just updating it.