Use of slugs as source_id could lead to duplication

[jamalex]

Slugs are being used here as source_id (e.g.), which is sometimes the best option we have, but noting that this could lead to some duplication here, as the same slug is sometimes used in different contexts, referring to something that is not "the same":
https://accessim.org/k-5/grade-2/unit-5
https://accessim.org/k-5/grade-4/unit-5

The "id" field may serve to provide better uniqueness than "slug".