[crabbone]

Why do you keep inventing stuff... No, JSON with duplicate keys is not invalid. The whole point of streaming is to be able to process data before it completely arrived. What "layering semantics" are you talking about?

This has no similarity with undefined behavior. This is documented and defined.

[kiitos]

A JSON object with duplicate keys is explicitly defined by the spec as undefined behavior, and is left up to the individual implementation to decide what to do. It's neither valid nor invalid.