Having unique page titles, and other page metadata, is supposed to help search engines give useful information to users. Hence, Google wants to punish sites that don't do so.
Google reports on Duplicate Title tags in the
Google Webmaster Tools. Simply navigate to one of your sites, then select Search Appearance and HTML Improvements. It gives a list of possible things you can do to improve the site's HTML to suit Google's purposes.
What does it mean to have a "Duplicate Title Tag"? The <title> tag occurs in the <head> section of the page. This value is displayed in the browser title-bar when viewing a page. There are also some <meta> tags like description or pagename that are used by search engines.
The idea is that these should be unique for every page on a website.
Often a site generates duplicate title tags because the software doesn't take enough care in some cases. For example there might be a page that's split over multiple sub-pages using a ?page=## parameter to select the pages. It's simple enough to add (page ##) to the <title> tag but not every software author thinks of this.
