Mongo is one of the few NoSqls to offer "unique" keys. While common in SQL, this is a big differentiator from other NoSqls and allows many use cases that would have otherwise ended up in the SQL world
Cassandra have this, any hash database should have some sort of unique concept or key, and IIRC graph database all node should be unique.
I didn't realize this was a rare thing in NoSQL world.
Nothing to do with the primary key and yes it is definitely rare in the NoSQL world.