Duplicacy of pages add no real content to the web and do nothing to improve the quality of the search engines output. By offering the same document over and over search engines would offer a horrible product with no variety. For this reason, duplicate pages are often filtered out. This is the exact reason Yahoo! dislikes many affiliate websites.
Duplicacy of content and pages are major cause for search engine optimization.Even if search engines like Google will not always penalizes you for duplicate content, they will still have to choose one of the URLs / pages to show in search results and ignore all other duplicates.

Photo via Sam Ul

If problem is related with WWW-pages and non WWW-pages URLs in the generated XML sitemap will then either be with or without www which may differ from the paths search engines such as Google.
To avoid duplicate content use canonical URL in wordpress.This is the definition given in Google’s Webmaster Central Blog a format that allows you to publicly specify your preferred version of a URL. If your site has identical or vastly similar content that’s accessible through multiple URLs, this format provides you with more control over the URL returned in search results. It also helps to make sure that properties such as link popularity are consolidated to your preferred version.
It is very easy to specify the canonical URL of a page – all you have to do is add this line in the head section…

The purpose of this tag is to prevent the problem of duplicate content.

The best thing about this tag is that all three major search engines(Google, Yahoo and MS Live) support this initiative.
header.php

Go to the theme folder in wordpress(wp-content/themes/
) and open the file header.php in your favorite editor. Now find the line…

and add the following code before that line…

<?php if ( is_singular() ) { ?>
<link rel="canonical" href="<?php the_permalink(); ?>" />
<?php } ?>

Now, it should look something like this…

<?php if ( is_singular() ) { ?> <link rel="canonical" href="<?php the_permalink(); ?>" /> <?php } ?> </head>

That’s it - you are done! Save the file and upload it to your server.

This fix will add the canonical URL tag to all the posts and pages of your blog. If you want to see a blog that have implemented this fix, take a look at the source of this page.
What is a canonical URL?

If you already know you would be surprised at how few webmasters have ever heard of a canonical URL. If you also spend time in Google Groups you will find people scratching their heads and not getting a clear answer as to how to troubleshoot and resolve this troublesome issue in Google.

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Tagged as: canonical url, duplicate content, seo