When we talk about duplicate content, we talk about important content blocks which belong to a same domain or which are shared among several domains, and that are identical or relatively similar.
In some cases, the content is duplicated on purpose between the domains, in order to manipulate the ranking of a website. Such questionable practices can have a negative effect on user experience, who will witness the same content repeating again and again in Search Engines Research Pages (SERPs).
Moreover, Google will impose you a penalty if it sees that you are using duplicate content in a wrong way or with wrong intentions. This penalty will reflect as a ranking disadvantage, or as a complete and definitive site removal from Google's index if you misused and abused of this practice.