XSD is only part of the value-offering... XSD + XSLT is the value provider for format heavy, robust, declarative, document transformations.
JSON-schema solves many of the validation issues of JSON in the absence of an XSD-for-JSON solution, to allow message processing and other validation assertions. From that perspective it's 'duplicated effort', but only to the extent that JSON messaging applications are maturing to the point they need the kinds of guarantees that were so clear during the development of XML/SGML. It's a whole different language supporting a range of JSON-powered clients that don't necessarily require transformation support or other guarantees.
This is about JSONs ecosystem maturing to the baseline, not about reinventing the wheel :)
It's not circular, it's what maturation looks like. Yeah, Json's great - lots of people are using it for lots of things. You could argue it's lack of being schema-bound is a great advantage, and you'd be right, and wrong, depending on your application.
This just serves to provide a way to say "yes, we can have strongly typed schemas as well as our nice succinct format" to those who need it.
