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Duplicate content can be likened to being at a crossroads where the signposts point in two different directions to the same destination: which way should you go? Even worse, the final destination is only slightly different. As a reader, you don’t mind because you get what you wanted, but the search engine has to choose which page to show in the search results because it certainly doesn’t want to show the same content twice.
Suppose your article about “keyword x” appears on :.exampleomkeyword-xand the same content also appears on :.exampleomarticle-categorykeyword-x. This scenario is not fictional: it happens in many modern content mexico mobile database management systems. Then suppose your article has been picked up by several bloggers, some of whom link to the first URL, while others link to the second. Duplicate content is your problem because these links all promote different URLs. If they all linked to the same URL, you would rank higher for “keyword x”.
There are many reasons for duplicate content. Most of them are technical: it's not uncommon for people to decide to put the same content in two different places without making it clear which is the original. Unless, of course, you cloned an article and published it accidentally. But other than that, it feels unnatural to most of us.