Duplicate content is content that appears on the Internet in more than one place (URL). This is a problem because when there are more than one piece of identical content on the Internet, it is difficult for search engines to decide which version is more relevant to a given search query. To provide the best search experience, search engines will rarely show multiple, duplicate pieces of content and thus, are forced to choose which version is most likely to be the original (or best).
It is the plague that infests the Interwebs, consumes bandwidth, search engine resources, space, time and storage and is a wasteful junk of human intelligence efforts to create genuine, original writings. Aka crap.
If you have already placed an content in the website or else if the same content exist in the website already then it would be showing as Duplicate content. Make sure whether you have placed the content before.