Report duplicated values in enum

[Amachua]

User story.
As an API Designer, when I define an enumeration with duplicated value, then I want that a warning is raised.

Describe the solution you'd like
When I use the following dummy.yml file

openapi: 3.0.0

info:
  title: Dummy title
  description: Dummy description
  version: 1.0.0

paths:
  /dummy:
    get:
      description: Dummy description
      parameters:
        - in: header
          name: some_header
          description: Some header description.
          schema:
            type: string
            enum: [dummy, dummy, not_so_dummy]
      responses:
        "200":
          description: All is good
        "400":
          description: All is good

And I run the command line

yarn spectral lint .\dummy.yml

Then I want to have an issue that is raised as the entry dummy appears too times in the some_header parameter.

Rule proposition

I've already prepared the following rule so that it could be included (if found useful), in the next version of Spectral.

  enum-duplicated-entry:
    type: validation
    severity: warn
    recommended: true
    formats: ["oas3"]
    message: "A duplicated entry in the enum was found. Error: {{error}}"
    given: $..enum
    then:
      function: schema
      functionOptions:
        schema:
          type: array
          uniqueItems: true

Should I open a Pull request to add it in your ruleset? :)

[P0lip]

Hey @Amachua
My apologies for the silence on the radio.
Yes, go ahead, please.
It's a neat rule. I like it.
We might need to tweak the given path a bit, to make sure we actually lint a portion of JSON Schema.

[Amachua]

I've started to work on this point, I hope I'll be able to open a Pull Request soon in your repository!

While working on this, what tweak do you think we'll need on the given?