Attribute option value uniqueness is not checked if created via REST Api

[hilrob]

Preconditions

1. Magento CE 2.1.4
2. PHP 7.x
3. MySQL 5.7

Steps to reproduce

1. Send the same call to create an option twice
   URL: /rest/V1/products/attributes/color/options
   POST:

{
    "option": {
      "label": "Black1",
      "sort_order": 9999,
      "is_default": false
    }
}

2. Get the attribute via API or check the backend
   Response:

[
  {
    "label": "offwhite/schwarz gestreift",
    "value": "281"
  },
  {
    "label": "red1",
    "value": "286"
  },
  {
    "label": "schwarz/mehrfarbig",
    "value": "282"
  },
  {
    "label": "Black1",
    "value": "287"
  },
  {
    "label": "Black1",
    "value": "288"
  },
  {
    "label": "Black1",
    "value": "289"
  }
]

Expected result

1. Only one option value is created
2. Second call must fail with exception

Actual result

1. Option values are created more than once.

[VasylShvorak]

@hilrob Thanks for reporting this issue. Created internal issue MAGETWO-70441 to investigate and track this issue.