With the SEO Plugin ge_canonical_XH you can insert a canonical link in the header of your pages, to avoid no wanted site-internal Duplicate Content. If you want to know more about canonical links and the negative effects of Duplicate Content for search engine ranking, search by google for “canonical link” or “duplicate content”, you will find a lot of informations in many languages.
Simply installation: just unzip the zip file and upload the folder ge_canonical with all files and folders to your plugins folder, that's all and should work.
Download: http://www.ge-webdesign.de/cmsimpleplugins/?Eigene_Plugins/ge_canonical_XH