As search engines try to find and index all the pages that they find on the Internet they rely on unique URLs as pointers to each piece of content. While there should be a single unique URL for each page on the Internet often web pages can introduce slightly varied URLs for the same piece of content. This resulting in duplicate URLs in the search engine's index. A common reason for this is the use of URL parameters. These are extra bits of data that are appended to the end of URLs and they can be used to do a variety of different things. Typically these extra bits of data are found in shopping cart software.
This tag is something that you add to your page that acts as an instruction for search engines. It tells them that no matter what URL might be showing up in the address bar make sure to index this URL as the primary URL for this content. Another way to clear up any confusion about how your site uses URL parameters is to tell the search engines directly through Google Webmaster Tools and Bing Webmaster Tools. Here, you can instruct search engines on whether or not they can ignore certain URL parameters.
Another reason that duplicate content may exist is because content may have been moved from one location to another on your site. The old location and the new location could potentially be in the search engine's index at the same time. To avoid this situation, whenever you move content around it's important to implement redirect rules. There are a few redirect types that you or your webmaster can use but let's take a look at two in particular. The first is known as a 302, or temporary redirect.
This should only be used for short-term content moves, like when you want to show an alternate page while your site's down for maintenance. It tells the search engine that the page it's looking for isn't there now but will be back very shortly. So, please don't do anything to your index. For long-term or permanent content moves which search engines are really concerned with you will want to use a 301 or a permanent redirect. These redirects tell a search engine that although they may have indexed a previous URL for that content the old URL is no good anymore.