Most of your website traffic is organic. And Search Engines do not like copied content. They penalize the websites for plagiarism. Googles Panda algorithm is an example of Search Engines algorithms that degrades the search ranking of a website if the content is copied.
What content must be unique
The content produced by your website must be unique. But what content. You should only be worried about the uniqueness of your posts or articles. Other items of your website like the logo, the navigations, the ads images, the ads text and the sidebars are not necessarily required to be unique. However the images used to describe the articles must be unique.
How to check for duplicated content
Internet is a source of knowledge. You learn a lot of things from using internet websites. Does this mean you cannot produce any content because it will be somehow copied? No, you can produce the content based upon your understanding or you can write about your opinion about a particular thing or perhaps you can share your experience.
Since you use internet very often, it is a great chance that some of your sentences might be very similar to other websites content. In order to check this you should use some tools like mentioned in this post. Or you can know that who else is copying your content and take some action to remove that content.
Free Content Plagiarism Checker Tools
Below is a list of tools that I have used. I will discuss them briefly here. Each one has its pros and cons. You should use the tool that suits you the most.CopySpace
It is one of the oldest and widely used tools for plagiarism inspection. They also provide the option to perform check for internal duplicate content. Moreover they have plagiarism alert service that is similar to Google alerts.
Grammarly
Grammarly is basically a spell check tool or advance grammar tool. But it also helps you to maintain the originality of your content. If you are not an English native then this is a good tool for you with multiple uses.