[Bug]: @defaultResponse model included in return type union generates anyOf with duplicate $ref

[happenslol]

Describe the bug

I wrap every return type in a Result alias, since it's not possible to define a global default error type, see #3082.

Since 1.7.0, this generates the following:

default:
  description: Server error
  content:
    application/json:
      schema:
        anyOf:
          - $ref: '#/components/schemas/DefaultError'
          - $ref: '#/components/schemas/DefaultError'

This is rejected by my OpenAPI codegen because of the duplicate indentical refs.

Here's what was generated on 1.6.0:

default:
  description: An unexpected error response.
  content:
    application/json:
      schema:
        $ref: '#/components/schemas/DefaultError'

The culprit seems to be @typespec/openapi3 v1.7.0 from what I can tell.

Reproduction

import "@typespec/http";
import "@typespec/openapi";
import "@typespec/openapi3";

using TypeSpec.Http;
using TypeSpec.OpenAPI;

@service(#{ title: "Repro" })
namespace Repro;

@error
model Error<Status> {
	@statusCode _: Status;
	message: string;
}

@error
@defaultResponse
model DefaultError is Error<500>;

alias Result<T> = T | DefaultError;

model Thing {
	id: string;
	name: string;
}

@route("/things")
namespace Things {
	@get op list(): Result<Thing[]>;
}

With the following versions:

"@typespec/compiler": "1.7.0",
"@typespec/http": "1.7.0",
"@typespec/openapi": "1.7.0",
"@typespec/openapi3": "1.7.0"

This generates the above with tsp compile ./main.tsp. Changing to openapi3@1.6.0 generates the correct version.

Checklist

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• The provided reproduction is a minimal reproducible example of the bug.