Google has been warning us for years against publishing duplicate content on the web. Why? It hurts the search experience when people find the same rather than unique information in more than one listing in the results.

Google’s rep at the 2007 Webmaster World Conference had this advice about Google and duplicate content.

Google wants to index the original source of all content. So, demonstrate to them that your content is the original by adding copyright information and by having the oldest indexed version of the content on the web.

Google wants to show one URL in their Search Engine Results Pages (SERPs) for one page of content. They do not penalize duplicate content, but do filter it out of the index so that it does not appear in the SERPs.

Google says that near-duplicate content may be interpreted as duplicate content and treated accordingly. So, truly make all of your content unique - changing just some parts of it may not be enough to keep it in the index.

Google says they have different types of filters for different types of duplicate content. This tells us that all duplicate content is not treated equally, so don’t make any unfounded assumptions.

If you use syndicated content, like RSS feeds, you must add value to your visitors. Without added value, it may be considered duplicate content.

Google says the same content in different languages is not duplicate. Anyone’s who’s ever thought about this already had that figured out, but it’s nice to hear it straight from the Search Engine’s spokesperson’s mouth.

 I found a few more items of interest on this subject in Google’s Webmaster Central Blog:

There, Adam Lasnik says: 

• “In the rare cases in which we perceive that duplicate content may be shown with intent to manipulate our rankings and deceive our users, we’ll also make appropriate adjustments in the indexing and ranking of the sites involved. However, we prefer to focus on filtering rather than ranking adjustments … so in the vast majority of cases, the worst thing that’ll befall webmasters is to see the “less desired” version of a page shown in our index.”

• Excerpts and occasional snippets will not flag your pages as containing duplicate content.

• If you syndicate your content on other sites, make certain they link back to your page containing the original. This show Google which content is unique and which is a copy.

• For boilerplate information that must appear across your site, include a very brief summary on each page that it applies to. Then, link to one unique page containing all of the details.