That is not considered duplicate content, Ezzaral. In fact, it is a huge boost to SEO to have duplication of many keywords throughout a site. It demonstrates to Google that the site is an "authority" on that keyword/topic.
"Duplicate content" refers to when multiple websites have the *exact* same content. i.e. the same article has been posted to multiple websites, or one website syndicates the RSS feeds of another, etc. Search engines frown upon this because if you do a search for something, and there ends up being a relevant article, you don't want the entire first page of Google results to be that exact same article (regardless of how great and relevant and useful it is) but hosted on a different domain. The main problem Google has with duplicate content is determining which source is the original and which website copied which. Publishers get frustrated with Google when they put money and resources into having writers put out great content. Then, some mashup website scrapes their content, reposts the article, and they show up in the Google results instead of the original publisher.