This should answer your question. Kat rose is asking the question and rand fishkin is answering
The following quote is taken from http://www.katroseconsulting.com/search-engine-optimization/attention-bloggers-should-you-be-worried-about-duplicate-content/
KR: Does reposting blogs on other’s blog sites hurt search engine rankings?
RF: It is possible that if you re-publish your content on another site, it could rank in the search engines rather than your own, original version. A link back is often enough to negate this, but there’s also a technical way to make it work 99.9% of the time and that’s with the “Rel = canonical” tag (http://googlewebmastercentral.blogspot.com/2009/12/handling-legitimate-cross-domain.html). If you use this (or have those blogs you guest post on use it), you can feel very confident that your own post will be the one to rank.
If you feel a little lost at the mention of rel: canonical tags, as many do, I’m going to break it down for you with the help of another great source—my friend and SEO guru, Tisho Richardson, Web Associate at ThomasNet.com (formerly the Thomas Register). Very simply, you put this type of html tag between the “head” tags on the blog post page of the ORIGINAL post; this way the search engines know that this is the one true original version and should rank this version above all others. This is an example of what a rel: canonical tag looks like behind the scenes of a website:
For example, with the first scenario I referred to–guest posting on another blog, the rel: canonical tag should appear on their site on the page where the blog is posted. In the second, where others are sharing your already published content, the tag should appear on your blog page (and always be sure that they link back to your original post).
If you have a wordpress blog, as I do, there is a plugin called “Canonical URL’s”. Once the plugin is installed, you will see a box under the post marked “Canonical”. In there you put the link to the post."