Drafter allows duplicate attribute and generates JSON Schema with duplicate required fields

[honzajavorek]

Following document

# Data Structures

## Struct (object)
+ field (required)
+ field (required)

# test [GET /]

+ Response 200 (application/json)
    + Attributes (object)
        + data (required, array[Struct], fixed-type)

will result in no user warning and in JSON Schema with "required": ["field", "field"]. I think this situation (basically user typo) should be handled more gracefully on parser level. See apiaryio/dredd#840 reported by @basavind for more details.