lint: should exit non-zero for a duplicate key name

A duplicate key is technically valid JSON [1], but recommended against (depending on the JSON spec) and is generally less portable.

But for our use case, any addition of a duplicate key is almost certainly unintended. So we should produce an error, rather than silently accepting it. I believe our Ruby JSON parser silently accepts it.

See also https://github.com/exercism/configlet/issues/312

[1] From section 6 of https://www.ecma-international.org/wp-content/uploads/ECMA-404_2nd_edition_december_2017.pdf:

> ### 6 Objects
>
> An object structure is represented as a pair of curly bracket tokens surrounding zero or more name/value pairs.
> A name is a *string*. A single colon token follows each name, separating the name from the *value*. A single
> comma token separates a *value* from a following name. The JSON syntax does not impose any restrictions on
> the *strings* used as names, **does not require that name *strings* be unique**, and does not assign any
> significance to the ordering of name/value pairs. These are all semantic considerations that may be defined by
> JSON processors or in specifications defining specific uses of JSON for data interchange.