If there are two pages on the web with essentially the same content, search engines will only show the original. This is completely reasonable: searchers want to be able to choose from among alternatives, not a whole bunch of identical content at different pages.
Google’s official explanation gives clear instructions about what to do when you have duplicate content for a good reason. That link will also give you details about the use of syndicated content. Here’s Bing’s word on canonical tags.
There are also sites that are created by scraping other sites and repackaging the content. This is deliberate misbehavior. But sometimes people buy a site that has been created in this way. There are also affiliate sites and e-commerce sites that are sold to unsuspecting buyers who don’t understand that the sellers are peddling identical sites to lots of people. This is sort of like buying a Rolex on a street corner. I’m sorry if it has happened to you, but there’s nothing you can do about it but toss the bad timepiece out and be more careful next time.
Having gotten the excuses out of the way, let’s talk about the reasons people usually end up with duplicate content.
First, they never really grasped the idea of plagiarism. I teach writing at a local college, and every semester I have papers that have content copied directly from websites. After they fail my class, these students presumably go on to write their company websites by pulling content from their competitors’ sites and sticking it onto their pages. Most of the students with this problem seem quite sincerely not to understand that plagiarism is wrong, and clients with this problem seem equally innocent. If this applies to you and your website, just accept that search engines prefer original content and give up using other people’s stuff, whether it seems wrong to you or not.
Second, they can’t write their own content. Not everyone has that skill. You sometimes see this in blogs where the blog’s author has written, “Here’s a great article I saw at thus and so.” Here, there is no attempt to trick human visitors into thinking that the material is original, so it isn’t plagiarism. However, it also isn’t new content. The right thing to do, from an SEO standpoint, is to link to the other site instead of duplicating their stuff.
Third, they’re writing about someone else’s content. If you’re analyzing a poem (and I have no idea why you’d be doing that at your business site, but bear with me), then you might need to reproduce the poem in order to be able to discuss it. You might quote extensively from someone else’s blog post in order to respond to it at your own blog. You might use a manufacturer’s description at your site, and then add your own thoughts — I don’t advocate that, personally, but I’ve been asked to do it before by retail site owners. In these cases, you’re quoting someone. Do it with proper attribution and make sure that most of the content is original, and you’re probably fine.
I actually have duplicate content issues myself because I moved this blog from its old home to this one and haven’t yet gotten around to moving all the images and shutting the old one down. This affects my PageRank and probably affects my search rankings, too. This true confession is just to say that I have compassion for people who have sites filled with duplicate content and feel daunted by the work involved in fixing the problem. It’s worth doing, though. And if you don’t yet have the problem, don’t create it for yourself.