Greg Grothaus of the Search Quality Team has posted a video (along with a presentation on the Webmaster Central Blog) covering duplicate content and multiple site issues that webmasters continue to face when trying to rank well in Google.
Greg begins by clearing up a popular myth about duplicate content, and that is that Google penalizes sites for having duplicate content. This is not the case. That's not to say that duplicate content can't have a negative impact on your rankings, but Google itself is not penalizing you for it.
Greg stresses that duplicate content is simply a factor on a "by query" basis. "What's actually happening, is that we're looking at the query that the user's doing, and we're saying that we want diversity in the results we're going to show a user," says Grothaus. He says those who think their content is being omitted because it is duplicate, will likely find that if they adjust their query to more specifically reflect the missing piece, they may just find that it shows up in results after all.
Google recognizes that most duplicate content is not created to be deceptive. There are of course exceptions, which are considered spam. Grothaus says even spam sites aren't being penalized for having duplicate content though. They're being penalized for being spam. Just like some spammers use bold tags, he says. They don't penalize people just for using them. And they don't penalize people just for having duplicate content.
Duplicate Content:
* example.com/
* example.com/?
* example.com/index.html
* example.com/Home.aspx
* www.example.com/
* www.example.com/?
* www.example.com/index.html
* www.example.com/Home.aspx
The above list from Grothaus's presentation shows examples of URLs that are different, but show the same content. Google will recognize that they're the same, and will try to pick the right one, (although sometimes they pick the wrong one). Greg says Webmasters are the best people to know which one is best, so it helps to only use one.
You will not be penalized for using more than one, but there are some issues that can arise that may negatively affect your rankings. For one, your link popularity will be diluted. Backlinks pointing to several different URL versions of the same content, will make it harder to accumulate link juice for one URL. Greg says that user-unfriendly URLs in search results may offset branding efforts and decrease usability as well. Plus, with multiple versions of the same thing, Google will spend more time crawling the same content, meaning it will have less time to go deeper into your site, and you run the risk of having content not get indexed.
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I really don't think this brings anything that new to the duplicate content question. It certainly doesn't 'bust' any myths. I mean the bottom line is that if you use duplicate content, you won't get ranked for multiple pages or sites. Google is basically saying here that your not going to get banned but its not going to help your overall cause. It's always a good idea to use unique content on different pages and if you have more then one site, you should not use the same content you used on the main site. You will never have multiple domains coming up for same set of content, it must be unique.