At the outset, please be assured that there is actually no penalty meted out to if your blog's content is found elsewhere in the web.
Let me go over case by case.
* A press release released by a PR site is redistributed or syndicated by several PR sites.
* Content syndication through RSS feeds is not frowned upon
* A news reported in BBC or CNN is appearing in hundreds of sites
I don't think there is any penalizing in such cases! Then why we are so scared about duplicate content when we are buying full PLR products that allow us to put our name as the author?
The only thing you must be aware in such instances is, your content won't appear in the top of search results which goes to the the same content that was published first.
In addition, seasoned social media marketers would laugh out if they come to know that bloggers still depend on search engine traffic. Man, those days are gone. Your blog can attract consistent and targeted traffic if you are effectively doing social media marketing.
Bloggers are often tempted to buy pre-made niche blogs with PLR license but are worried about duplicate content penalty. You can buy readymade niche blogs with full PLR and publish it as such or do at least change the title and the first two sentences in the blog post. Premade niche blogs come with premium plugins and themes.
If you want high quality ready made niche plr blogs, please let me know. I would sell it with 100% unique content.
I know you are still skeptic. How about hearing it all from the horse's mouth?
Read this excellent and very detailed post from official Google Webmaster Central Blog..
Demystifying Duplicate Content Penalty