Ther is tons of duplicate content out there. All over the place.
Take a look a some of the SPAM Adsense sites out there. RSS feeds you name it. Not to mention like edmc said what about joke sites, quote sites all that stuff is the same.
The point is does it hurt you?
I have to say it can but if you are good it wont. I have proof of this because I've done it many times. One of my old sites started of with nothing but articles that were given to me from other sites. I quickly created proper SEO and even added some external links to those pages. I was quickly on top of Google for keywords in those articles and I surpassed the the sites that gave them to me to use. I can say that about 5 of those articels were indexed in Google on about 20 other sites. I was #1 of course for most keywords because I was better at optimizing my site than they were.
There is a fine line here about having duplicate content, but it's not always bad or how would RSS
blogs, news feeds, auto generated SPAM pages even be indexed. Why do they rank high in Google? Good SEO.
If you take the exact same article and give it to 2 sites and one lets Google find it and list it and that's all. Then the other adds good internal links to the page, adds external links to the page, optimizes the keywords and adds proper tags, the second one will be on top of Google regardless if the first one was indedxed first.
Just think about. Plus it's so hard to have "EXACT" copies as SE's read the code. So the code in your template or page will throw off an SE in most cases as well. Add an H1 tag a few h2 tags. Now it's not the same.
I would not worrry about dup content all that much. I would be more worried about giving your content out so others can beat you in the serp's