I guess before we talk about why duplicated content is a bad thing we should probably explain what we mean by duplicated content. It’s really simple, any text that you find elsewhere that you copy and paste into your website is classed as duplicated content. It harms your search engine results (even if Google confused us all lately with a video that told us they do not punish duplicated content).
So okay Google does not directly punish your website for doing so but guess what? It checks your website, see’s that the content is the same as elsewhere and it omits that particular page or article on your website, so simply put if your whole website is made from duplicated content it will get omitted. So it might be okay to use a little bit of duplicated content though we would always advise completely against it.
Have a look at this little diagram that explains how Google handles new content when people search for it.