There is a brand new Google Webmaster Tools error for Sitemaps files. The new error or warning is named “Duplicate URL” and has been springing up for many webmasters recently.

The full error reads:

The first reports of such an error being received by webmasters was on June 29th, Wednesday. The error basically tells the webmaster that they have the same URL (a duplicate) listed in the Sitemap file and they should probably remove it. In some cases, Google is wrong about what is duplicate and what is not. For example, if domain.com/page/ is technically a different page from domain.com/page/index.html, Google will still consider that a duplicate listing.

JohnMu from Google explains in detail:

We have lots of forum threads, all from Google Webmaster Help.