On Wordpress they introduced the canonical meta tag, this is designed to instruct the search engine bot which URL to count as the primary source of the content. But the snippets should get discounted anyway since their pages have less content on them. The problem comes when the category and tag pages are very similar to the main post page i.e. they are replicating the full post content. These pages should ideally use the excerpts function in the template.
My forum signature is under construction.