That compounds really badly with another lack of specificity in JSON: parsers can treat duplicate keys in arbitrary ways, e.g. keep only the first or last value

That compounds really badly with another lack of specificity in JSON: parsers can treat duplicate keys in arbitrary ways, e.g. keep only the first or last value, combine values somehow, error out, etc. When keys are not unique, their order can matter again, and can matter differently to different parsers.