This means if you wish to have high rankings in Google you need unique content, so do not use copy paste from other websites, if you somehow need to use duplicate content then try to have better inbound links than website from which you are taking duplicate content.
Content could be duplicated by many various reason.
Let see top 5:

1. The content of a web page is available in different formats: web page, printable page, PDF, mobile phone page, etc.

2. The content of a web page is syndicated, for example news articles or blog posts.

3. The content management system (CMS) displays the same content in different locations. For example, an item might be listed in a “Size” category and in a “Color” category.

4. The website owner offers mirrors to make sure that a website does not slow down when many people want to access the same page at the same time.

5. Someone stole the contents of a web page to reproduce it on other websites.
Google is very serious in detecting duplicate content and they improve they patent application to detect duplicate content very often.

What Google do when… Read the rest

1. Google making changes to SERPs
2. Google and Yahoo make a deal
3. Google vs Groovle 0-1