One of the core features of the Google Panda algorithm is penalising sites which contain duplicate content. Duplicate content makes it difficult for Google to rank the correct for the users search query.
What amount of duplicate content does Google consider harmful?
Also known as “near duplicate content”, it is content which has been edited to read differently but still gives the same content. Below is what Gary Illyes had to say:
Basically, there is duplicate content between different websites and inside your own website.
Gary Illyes – Twitter