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:
This URL is a duplicate of another URL in the sitemap. Please remove it and resubmit.
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:
This is a new warning message we added to inform you of this issue. This does not mean that we treat your Sitemap file in any way differently, it's just for your information (and this information wasn't shown to users before). In general, it makes little sense to submit duplicate URLs in a Sitemap file, which is why this message was added.
There is one item which may lead to confusion here though - Google's Sitemaps processing generally simplifies URLs in ways that make sense on a whole. This includes removing "/index.html" from the URL if that's the last part. In general, that makes sense, since you want to show users the relevant part of the URL (and since most servers will automatically check "index.html" when the root page is requested).
However, there are some cases where "/index.html" is a relevant part of the URL and can't be removed. In these cases, we'll generally discover those URLs through our normal web-crawl. At the moment, it's possible that our Sitemaps processing will show these warnings in those cases - but since this does not signal any change in the processing of your Sitemaps files, you can safely ignore these warnings.
In short: these warnings are new, but the processing of your Sitemaps files has not changed.