code: func ValidateListUniqueStrings(v interface{}, k string) (ws []string, errors []error) {
  	for n1, v1 := range v.([]interface{}) {
  		for n2, v2 := range v.([]interface{}) {
  			if v1.(string) == v2.(string) && n1 != n2 {
  				errors = append(errors, fmt.Errorf("%q: duplicate entry - %s", k, v1.(string)))
  			}
  		}
  	}
  	return
  }
docs: // ValidateListUniqueStrings is a ValidateFunc that ensures a list has no
  // duplicate items in it. It's useful for when a list is needed over a set
  // because order matters, yet the items still need to be unique.