Today i went to Google Webmaster Tools to check for duplicate meta description.
On almost all my news pages, this is true cause my rss feeds links to the news piece with a parameter (?rs=rss) so i can track my traffic from rss feeds.
I thought the following snippet would prevent this.
<link rel="canonical" href="http://www.example.com/section/textid/">
What am i missing to prevent Google Webmaster Tools to report duplicate meta description ?
You should go to the URL parameters setting in WMT and set the variable rs to be ignored, by indicating that it does not change page content seen by users.
Note that rel="canonical" is just a suggestion and not a directive. Google can choose to ignore it if it feels it's been incorrectly added or wrongly implemented.