Yes unique content is very important for your article and google count it. Unique content is help on Quality, original articles that contain useful and interesting information build your credibility with visitors, creating trust and expanding your reader base. Their are no percentages defined for this. But google has some algos going behind which sees this things.
Duplicate content would definitely be banned by the Google Search engine someday so its good to use fresh content everytime instead of duplicate. If you cannot write unique content all the time, you can use some content spinning tools that would mould the content with less effort. Some tools are as follows: http://www.ezrewrite.com http://freearticlerewriter.com http://article-rewriter.appspot.com http://www.wordhippo.com Hope this would help you. Thanks&Regards Daniel