Fauna DB - Create Document with ref to Document of same type failed - One-to-many

Question

I'm trying to create a new Document with a ref to a Document of the same type (Content sites, which can have childrens or parents of each other). I'm getting the following error message:

description: 'document is not unique.',requestResult: RequestResult {
method: 'POST',
path: '',
query: null,
requestRaw: '{"create":{"collection":"Content"},"params":{"object":{"data":{"object":{"title":"Neu 3","content":"<p>Content</p>","type":{"ref":{"collection":"ContentType"},"id":"340867692647940301"},"author":{"ref":{"collection":"User"},"id":"337159230796595404"},"date":"2022-09-05","time":"20:16","published":true,"parent":{"ref":{"collection":"Content"},"id":"341985613786906830"}}}}}}',
requestContent: Expr { raw: [Object] },
responseRaw: '{\n' +
  '  "errors": [\n' +
  '    {\n' +
  '      "position": [ "create" ],\n' +
  '      "code": "instance not unique",\n' +
  '      "description": "document is not unique."\n' +
  '    }\n' +
  '  ]\n' +
  '}\n',

This is my GraphQL schema:

...
type Content {
  title: String
  content: String
  type: ContentType @relation
  author: User @relation
  date: String
  time: String
  published: Boolean
  parent: Content
  visibility: ContentVisibility @relation
  revisions: [ContentRevision] @relation
}
...

and here is my FQL part:

 q.Create(q.Collection("Content"), {
                data: {
                  title: title,
                  content: content,
                  type: q.Ref(q.Collection("ContentType"), type),
                  author: q.Ref(q.Collection("User"), author),
                  date: date,
                  time: time,
                  published: published,
                  parent: q.Ref(q.Collection("Content"), parent)
                }
              })

The idea is, to create subpages:

Site-1 (type: Content)
  - Site-2 (type: Content)
  - Site-3 (type: Content)

I have no idea what's going wrong here - type and author is working, why the parent not?

Thanks