[masklinn]

Both are only "correct" for specific implementation, this is not specified behavior (and duplicate keys is strongly recommended against by the key)

[_ZeD_]

absolutely. This is nothing more than a clever trick, but I would never rely on it.

Honestly, tough, I think all major JSON parser behave following the two assumption.

[masklinn]

streaming parsers can't follow the assumption short of becoming useless. They're either going to send only the first instance or going to send two different events.