text: Title: `items` of `ListBlock` in `StreamField` have same `id` as the `ListBlock` itself
  Body: Hi, 
  
  I was just working with a `ListBlock` in a `StreamField` and noticed an error message in the developer console.
  ```
  Warning: Encountered two children with the same key, `97c098cb-2ad1-445f-9ea7-36667656108c`. Keys should be unique so that components maintain their identity across updates. Non-unique keys may cause children to be duplicated and/or omitted — the behavior is unsupported and could change in a future version.
  ```
  
  Running a query on the GraphQL endpoint reveals that all the `items` of `ListBlock` in `StreamField` have same `id` as the `ListBlock` itself (and thus all the `items` have the same `id`). 
  ```graphql
  query {
    page(slug: "first-article") {
      id
  		title
      url
      ... on BlogPage {
        freeformbody {
          ... on ListBlock {
            id
            items {
              id
            }
          }
        }
      }
    }
  }
  ```
  ->
  ```json
  {
    "data": {
      "page": {
        "id": "5",
        "title": "First Article",
        "url": "/articles/first-article/",
        "freeformbody": [
          ...
          {
            "id": "97c098cb-2ad1-445f-9ea7-36667656108c",
            "items": [
              {
                "id": "97c098cb-2ad1-445f-9ea7-36667656108c"
              },
              {
                "id": "97c098cb-2ad1-445f-9ea7-36667656108c"
              },
              {
                "id": "97c098cb-2ad1-445f-9ea7-36667656108c"
              }
            ]
          },
          ...
        ]
      }
    }
  }
  ```
  
  Not sure what downstream error this might cause, but I guess the React warning will have it's reasons 😉