Currently one of the most frequent questions I am asked is - Does duplicate content really matter? Just for a change I can give a definitive answer - As far as Google is concerned at the moment; Yes!
As with all these questions what is really meant is does this matter to Google and we can see from Matt Cutts blog and the Google Webmasters Blog that it's not just a myth, You will be pushed down the rankings for doing it.
Google actually identify duplicate content down to short quotes, if you don't link it to the originators site as a quote you *will* be marked down and the content will have a lower ranking in the Search Engine Results Page (SERP.) THere is a short YouTube Video about it.
Overnight this has destroyed the concept of article "syndication" where a journalist or copywriter would write an article one and send it to multiple publications, being paid for each one. I have seen writers weep, on stage as they have recanted this. I feel for them, but that's how the internet has changed life (I was fed up with reading the same article in different motoring magazines anyway!)
The problem with all of this is that it is retrospective. All those old news articles on your high authority site? check them as they can suddenly be marking you down. Those datasheets that just said what was on the main article? The old ideas that if you keep saying the same thing over and over on different pages in your site to just to drive up the rankings? Now we all knew it was 'cheating' now you get penalised for it.
A really common mistake I am coming across on even large sites is page replication across domains and URL's. I thought this was obvious and people knew, but I will spell it out... Having www.mysite.com and www.mysite.com/home as the same page is duplicate content. I have tested it, having removed the page and redirecting all the incorrect links and watched the site climb up the search rankings over the next few *days* This really seems to make a difference!
The other really simple mistake is buying all the domain names that relate to you and pointing them at your site. www.mysite.com having the same content as www.mysite.co.uk *is* duplicate content! It seems really obvious, because it is! but if you must have multiple domains redirect them, better still make them relevant to the country/business type. Best advice, if the domain doesn't relate to you, don't buy it. You may not want www.mysite.xxx as a porn site, but knowing that xxx is blocked as porn, do you want it pointing to your home page? Leave it alone!
If you are worried, get somebody that knows what they are doing to look at it. Just fixing the stupids will make a remarkable difference!