When using Wordpress and All-in-one SEO, I typically check the 'no index' option for categories to avoid duplicate content. However, if the category page contains multiple posts, do you think it's still necessary to have a 'no index'. If a wordpress category contains many posts, is it still recommended to check 'no index' for categories on the all in one SEO pack? I know we want to avoid duplicate content.
I'm presuming you're referring to the category base here, in other words, yourblog.com/category/category-name/ You want to remove that completely using Brian Shoff's "Category Base Killer" plugin (he has a 1.5 in the works which I've been privately testing for him that's going to blow his 1.0 version and Top Cats out of the water). If you're not referring to the category base, then just use the_excerpt(); in your Theme's archive.php template file.
I personally set it to nofollow as I am only interested in my content pages being indexed. Yeah they show as duplicate content but you will not get penalized because of it.
I think you can leave it like that. Search Engines over the time developed and can deduce they are listing page and will not penalize for the duplicate content. I don't rank or do well on my category pages but on page specific posts i rank well. If you are scared then you can make them noindex so that google will only index the post pages.
I think it's good that you're being cautious. Most of my blogs are based on Google searches. I wouldn't risk it.