uniqueItems (value, schema, opts) {
    if (value && value.length && schema.uniqueItems) {
      const length = value.length
      let item, i, j
      // Check n - 1 items
      for (i = length - 1; i > 0; i--) {
        item = value[i]
        // Only compare against unchecked items
        for (j = i - 1; j >= 0; j--) {
          // Found a duplicate
          if (utils.deepEqual(item, value[j])) {
            return makeError(item, 'no duplicates', opts)
          }
        }
      }
    }
  }
Validates the provided array values are unique.
Validation succeeds if the items in the array are unique, but only if the value of this keyword is true
see http://json-schema.org/latest/json-schema-validation.html#anchor49

@name Schema.validationKeywords.uniqueItems
@method
@param {*} value Array to be validated.
@param {object} schema Schema containing the `uniqueItems` keyword.
@param {object} [opts] Configuration options.
@returns {(array|undefined)} Array of errors or `undefined` if valid.