By default WordPress is designed to file the same content in multiple areas archives, categories, and tags.
The idea was to enhance the usability of a blog for readers. This way, people could easily access information from various points.
At the same time, this structure can appear as a useless, duplicate content to the search engines. Even worse, the strength of your internal links are diluted.
Pick only one way for the search engines to follow be it categories, tags or archives. To change this, go to Settings > All-in-One SEO and look for the "Use noindex for Categories," "Use noindex for Archives" and "Use noindex for tag Archives." The one that you do not uncheck is the one that will be used by the search engine index. I usually uncheck the Categories and check the other two.