`class` duplicates after `rehypeStringify`

[kuboon]

import { toHtml } from "npm:hast-util-to-html";

import { assertStringIncludes } from "@std/assert";

Deno.test("remarkTransform adds id and anchor to headings", () => {
  const html = toHtml(<div class="test">Hello World</div>);
  assertStringIncludes(
    html,
    `<div class="test">Hello World</div>`,
  );
});

AssertionError: Expected actual: "<div class="test" class="test">Hello World</div>" to contain: "<div class="test">Hello World</div>".