This is a very basic issue but one that we see crop up regularly.
Duplicate content, in very simple terms, is to be avoided. This could be a page that has been used elsewhere on your site or a very keen Webmaster buying every domain extension of your brand and replicating the content on each.
We sometimes have a situation where a client purchased a domain and then through a branding initiative or other strategic reason, have gone on to buy a second domain. This could be a different name or just a change of extension, e.g. .com to .co.uk.
Either way, if not properly managed, this could lead to a duplicate content penalty and one or all sites could be negatively affected.
So what should you do? If you’re dealing with a potential issue of duplicate content within the same site, e.g. landing pages used for Pay per Click, these should be excluded from the natural search results by use of your robots.txt file.
If you have moved from ‘old.domain’ to ‘new.domain’ you should implement a 301 re-direct, i.e. permanent redirect in your .htaccess file. A 301 can also be used to manage ‘old.page’ to ‘new.page’ and acts basically as a sign-post informing the search engines that there has been a change. If used correctly, as well as re-directing the visitor to the correct destination, other factors such as in-bound links and search rankings should carry over to the new location.
Of course this only covers inadvertent duplication. If you have borrowed content from elsewhere on the Internet, be aware that Google will find it and could punish you as a consequence. Whether it is just a blog post you liked or the entire body text from a competitor’s website, you could see your rankings slip as a consequence.
Duplicate content adds no value to your site. Whilst visitors might be oblivious to it, the search engines certainly are not. It won’t add to the individuality of your site and will do little to maintain any kind of running theme. Your best bet is to always start afresh and produce unique content that is targeted for your site’s visitors.
There is an excellent article at Google Webmaster Central Blog that discusses dealing with duplicate content in more detail.