ExJsonSchema.Validator implementation for "uniqueItems" attributes.
See: https://tools.ietf.org/html/draft-fge-json-schema-validation-00#section-5.3.4 https://tools.ietf.org/html/draft-wright-json-schema-validation-01#section-6.13 https://tools.ietf.org/html/draft-handrews-json-schema-validation-01#section-6.4.5