How to do complex deduplication

[gwenwindflower]

With dbt we have the power to decide what aspects of an entity matter, and with that, we can find ourselves dealing with situations where duplicates are not as straightforward as every column matching in 2+ rows. This guide will explain how to deal with these situations.

Transferred from the archived devblog repo, specifically this issue

@lbenezra-FA is working on a draft from this outline