This is helpful, from Matt Cutts:
I often get questions from whitehat sites who are worried that they might receive duplicate content penalties because they have the same article in different formats ( e.g. a paginated version and a printer-ready version). While it’s helpful to try to pick one of those articles and exclude the other version from indexing, typically a whitehat site doesn’t neet to worry about 1-3 versions of an article on their own site.
In other words, if you have 2 or 3 versions of the same article on your own website and it doesn’t exist anywhere else then you don’t have anything to worry about with duplicate content. That’s because the search engines index the best version of an article or web content. That’s why you can have the original article, a .pdf version, another version for easy printing, etc. You won’t have to worry about penalties. That’s a very important distinction.
Even more importantly, I think, you need to seriously consider how much you syndicate your content. Here’s why:
However, I would be mindful that taking all your articles and submitting them for syndication all over the place can make it more difficult to determine how much the site wrote its own content vs. just used syndicated content. My advice would be 1) to avoid over-syndicating the articles that you write, and 2) if you do syndicate content, make sure that you include a link to the original content. That will help ensure that the original content has more PageRank, which will aid in picking the best documents in our index.
That’s a very important piece of information. These submission services that promise to submit your content to thousands of websites at one time may actually be hurting you. When it comes to content syndication, more is not necessarily better. That’s not to say that it’s better not to syndicate at all. There is an optimal level of syndication. For search engine optimization purposes, it is in your best interest to be weary.
I would encourage you, if you’re going to put content on your own website AND syndicate it, to put it on your own website first then wait a couple of weeks before you syndicate. I would also limit myself to a handful of article directories and websites. First, I’d pick the websites that I know I’d like to see my content on and I’d submit to them. After a couple of weeks (again), I’d submit my content to no more than a dozen directories and/or submission sites. I would stay away from the submission sites that submit to thousands of sites at once. The only exception would be if my content was in a niche that would intrinsically limit who will publish it and then I’d probably submit only to those mass submit sites. It’s usually better to submit only to mass submit sites or specific directory sites, but not both. The main reason for this is because any article or content that is syndicated to more than 100 places online has likely reached its limit.