It’s not just duplicate content that will affect where you rank it seems but Matt Cutts of Google’s Webspam team also wants to see you bring additional value. Based on interviews with him on the matter if you have the same content as another site just written differently Google will try to catch this and while based on other factors you might have ranked second to another similar page you could get knocked down off the first page.
If your SEO should have you on the first page and you can’t work out why you’re not this is something to look at then: for your SEO to work you want truly unique content and it is of course quite possible that you don’t even realize that other sites have happened to write very similar content to you; it could even be that they have copied your article, not duplicated but rewritten, but still come out ahead of you because their SEO is just a little better.
It may seem like a lot of work to go through each page and check if for certain key terms other pages have similar content but it could be worth doing: the other option of course is to focus your SEO on increasing Pagerank and other factors and you could suddenly jump up the rankings and other sites could fall right down.
