One way in which duplicate content can be generated is by article submissions. The same article is published on many different article directories, and also on any amount of websites that chooses to copy it as content. Google make it clear that if you have the same article on your website, then it might not be yours that is retained. You are best to use a different form of the article on your own website than you submit in order to avoid this. If you do not use the article yourself, then you have nothing to worry about.
What will happen in that case is that the Page Rank you accumulate through all of these links back to your site will slowly reduce as more pages with the same article are filtered out. Ultimately this could affect the search engine listing and PR of your own web page. There is little you can do about this other than to keep submitting articles to continually generate new links to replace those that are being lost. Incidentally, always submit your articles in .txt format, so that when they are republished they will be reformatted and appear different to the spiders.
If you think that another website has scraped or stolen your content, you should report it. Google will likely spot this and realize that there is more scraped content than only yours on the site and immediately drop it from the listings. Keep in mind that Google will retain the version that it deems most appropriate for the search terms being used, and that might not necessarily be yours. Yours might reappear for another search term.
If you learn how to avoid duplicate content, and how to check that there is none involving your website, then you can avoid your web page listing from suddenly disappearing from its first page listing for no apparent reason. If that happens, check for duplicate content and if you find it then you should make a complaint to Google.
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