Site Manager - XML Sitemap error

When adding typical xml details such as:

<?xml version="1.0" encoding="UTF-8"?><urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">   <url>      <loc>http://www.example.com/</loc>      <lastmod>2005-01-01</lastmod>      <changefreq>monthly</changefreq>      <priority>0.8</priority>   </url></urlset>

We get the following error displayed:

Validation failed: Slug Duplicated page with slug=sitemap.xml format=xml method=get in ["views/pages/system_pages/sitemap.xml.liquid", "pages/system_pages/1.liquid"] - (see DevTools console for more details)

This is occurring on both my test site and Grahams.