A threat to making content king
Search Engines do not like duplicate content indexed. If you have many similar pages and you are not flagging them, these pages will not rank well and make your indexation of key pages tricky. Duplicate content can either be confusing for users (and indeed search engine algorithms); it can also be used to try to manipulate search rankings or win more traffic. As a result, search engines aren’t keen on it, and Google and Bing advise webmasters to fix any duplicate content issues they find.
Identification of duplicate content
A crawl of your site will help Identify duplicate. It is important then to decide on the master page, the page with most internal links or/and with the best UX.
You can fix duplicate content issues by: Preventing your CMS publishing multiple versions of a page or post. Using the canonical link element to let search engines know where the ‘main’ version of your content resides. Canonicals are best used when you have very similar pages for instance same t-shirt in different colours. You don't want to index the same t-shirt for each colour, although you want to keep the urls for the user to browse through the options. Beware though that sometimes canonicals are ignored by Google so it is important to monitor the results of canonicals implementation and follow up with a new strategy if necessary.
Same language, different territory target
A possible cause for duplicate content is when targeting different territories (UK and US for instance) with different domains or urls but with a very similar content. Make sure then that you have well implemented hreflang!