If you're trying to add a new page to super but you get an error that the page is already added, this means that super has already found and added the page to your site. But where is it? Super will add the page where it first finds it. Let's find where that is.
Take the last part of the Notion URL:
And add it to your custom domain
If Super has already added it to your site, then it will rewrite the URL as
https://your-super-site.com/content/some-page-on-the-site or whatever it is.
If Super does not change the URL, then another page is using that title/path at that location.