Duplicate content isn’t just a problem from a plagiarism perspective, it also has a negative effect on a website’s SEO.
Search engines use content to decide which website is the best match to the search query to show in the search results. Duplicate content confuses the search engines on which website it should show in the results. If multiple websites have the exact same answer, which one should it give credit to? The result is that all of the websites using the same content get lowered, while another website with unique content will get ranked higher because it will be seen as more valuable for the search query.
How does Google handle duplicate content?
Not all duplicate content is inherently bad and Google is pretty good at figuring out the difference between normal duplicate content and spam. Google’s Matt Cutts explains: