There's a lot of confusion about duplicate content. For example, people worry that submitting the same article to a few different directories will have negative ramifications for the URL they're linking back to in the author bio-box.
But this is a false concern. The main thing to remember is that you should keep all the content on your domain unique. Then you should be fine.
And you should also keep an eye on comments posted to your blog. This is because if someone posts content that has been lifted verbatim from elsewhere on the web that's duplicate content, and it's now on your domain. This blogger had such a problem and it had a substantially negative effect on his search engine traffic.