First thing right off the bat. Lets dispel the big myth just between us before we even get started.

There is no, I repeat, no duplicate content penalty. I know someone is going to want to argue with me on this, but, trust me, you're wrong.

Consider what a penalty is. A penalty means you lose something. You get set back. You net less than you originally had.

When you keep this in your mind and you look at what happens with duplicate content of any stripe, you'll see that there is no penalty in play.

That finished, we'll move on and consider the possibility that there is indeed a duplicate content problem and if there is, what we can do about it.

There are two distinct situations to consider when talking about duplicate content; off site duplicate content and on site duplicate content.

Off site duplicate content occurs when in whole or in part another website on another domain has content that is similar to such a degree that it can be considered duplicate. There isn't a whole lot you can do about this situation unless you are the originator of the content and the other site is using your content in violation of your copyrights (this is another discussion for another thread).

Short of that, if you are the originator and you are allowing other sites to use your content, you want to be sure that you require some link attribution for that use. Ultimately the content on your site will rank for the content because it is the originator. But this is only possible if the search engines can determine who the originator is. without the link attribution, there is no way for them to make that determination.

If you are using some one else's content (WITH their permission should not need to be stated here) you should in all cases attribute them as the originator of the content. You should also have some reason for using the content that has absolutely nothing to do with getting traffic from the search engines.

The reason for this is that the search engines do not want to have the same article showing up several times in the first 10 or even the first 1000 listings. If the first 10 results for a search were all separate copies of the same article, how often would you search at that search engine? Not much, right? Same for every one else.

So rather than a penalty for all of the other pages that have the same content, what the search engines do is give a bonus to original (or originating) content and position it higher in the results. the copies are left to sink further down for that search, in favor of the originator, or in the case that that can not be determined, the site with the strongest case for relevancy and authority.

But there are still reasons to include content duplicated from other sites on your blog. Mainly because offering the content to your readers locally makes more sense than sending them somewhere else to find it.

The second case happens a lot for blogs. Blogs, by their very nature and architecture have several pages that will display very similar or even identical content. This is not as much of a problem as you have probably been led to think it is.

First thing is the idea that duplicate content on your blog will throw some of your pages into the supplemental Google index. This is false. Matt Cutts has said that low page rank, not duplicate content lands you in the supplemental index.

Another thing to be aware of that Matt has said is "Google knows what a blog looks like". Since Google also demonstrably likes blogs, one could reasonable infer that Google is not going to "penalize" your blog for having duplicate content.

What they will do however is select one of those pages to display in the SERPs and the others will sink down for any give search. This will not prevent your blog from positioning well for that phrase, and indeed those other pages may back up the positioned page for it's ultimate position, it will simply prevent your blog from positioning several pages in the SERPs with essentially identical or duplicate content.

In my experience, Google does a good job of selecting pages, but there are things you can do to get even more pages with even better positioning by reducing the duplicate nature of pages.

The technical discussions of these methods is beyond the scope of this article, but the choices amount to limiting the number of pages you index to ensure that the pages you want to position well are the one's selected by Google, or differentiate those pages by offering unique perspectives on the same content.

There are spinners available now that claim to do this for you, but I caution you that thesaurus spinners tend to take a little bit of good content and turn it into a pile of crappy content.

Another thing to do that I do endorse is to limit the number of categories and tags you use for each post. One category and 3-5 tags is a good base line and then you can experiment from there.

Finally, I highly recommend a new technique I developed and have been using with good success on some of my blogs. Essentially you create a new distinct and original title and description for each tag, each category, the front page and your feeds for each post you post to your blog.

This method has two great advantages. First it will automatically force you to limit and to select carefully the category and tags you use for your post, and second it is work, so if you do it, you will enjoy an advantage over the vast majority of people who want a crappy push button solution.

On my main blog I posted about why you might want to adopt a unique tag system
http://danemorgan.com/duplicate-content-problem-solution-idea/63/

And how to do in WordPress
http://danemorgan.com/unique-content-tags-categories-wordpress/64/

And I posted another article on how to tag your posts for better results
http://danemorgan.com/blog-tagging-tags-traffic-promotion/65/