i am doing some seo for a client website
i have about 30 pages which have duplicate content so i put a canonical to all 30 pages linking to the main page
but on some of the pages google is showing the duplicate pages
what can i do

Verify that your redirection of cannonical content is actually working; this will include ensuring your htacess is correct and all of your links are pointing to the correct version of the page. If it is then just be patient. Depending on the rate google is crawling the website it may not be doing full crawls or crawls on a regular basis. Already Google on a highly crawled page will be 3 days behind (changes you make today wont show up for 3 days in your webmaster tools). So on less frequently crawled pages this could take a month for the changes to reflect accurately in your webmaster tools.

To try to expedite the process; you can try building links to the correct version of that page through social media/web 2.0/etc. in an effort to get Google to crawl that correct page. Also identify the pages that are crawled the most frequently and work on improving the links to the correct less frequently crawled pages.