Deduplicate origins in the GitHub lister
In some circumstances, GitHub will return two separate repos with the same html_url in the same page. This makes the lister fail with a cardinality error.
Sentry report: https://sentry.softwareheritage.org/share/issue/642ff21b110740249c4b4bd1c0611be4/
Test Plan
a patched GitHub lister can continue on
Migrated from D6716 (view on Phabricator)