No I didn't miss it, I just don't think that duality is inherent to OOP and is pretty much a byproduct of OOP languages being imperative, mutable and referential, which makes OP conflate all those properties.

What do you think prevents OOP languages from implementing automagical non-referential equality by data traversal by default like FP languages do? Is it some property of OOP itself? IMHO it's not a property of the paradigm.

The duality is still present in FP languages, it's just hidden under a nice automagical layer.

    (= (cons :a (cons :b '()))
       (cons :a (cons :b '())))
     => true

Both lists are still separate memory regions. They're the same data only in a platonic way and there's nothing that prevents OOP from implementing the exact same abstraction.