Thanks +Thomas Pfeiffer
Usually, embedded documents are specific to the parent document, like different addresses for a customer document, so duplication and consistency aren't too much of an issue.
Consistency and duplication is only an issue, when you have to share parts of related data in order to generate quick reads of the data. If this data is also stored in other collections and that data is also very volatile (changes often), you need to do a lot of heavy lifting to keep things in order. So, when you do have that kind of issue, you avoid it by only using references between the documents and yes, have application level joins.
So, it is interesting that ArangoDB has joins as a document store. "How are the joins horizontally scalable?", would be my next question.