The hook currently clears duplicated records from the included collection, but as this SO post says, data can have a record which is also present in included because it is part of a self-referenced relationship.
I'm awaiting further oppinions about this to see what should the post-serialization process should do in this case.
- Discussed it on Ember's Slack channel, shouldn't cause any troubles on Ember Data. Possibly the JSON API spec maintainers haven't ruled on this case.