I am always on the look for duplicate content issues. I think I have seen -50 positions for nothing more than a lot of duplicate content although I am looking into other possible issues. Generally speaking, Google will identify the best pages on your site if you have a decent on-site architecture. It’s usually pretty decent at this but it totally depends on where you are linkbuilding to within the site and how your site navigation is put together.
Don’t invite duplicate content issues. I don’t consider it a penalty you receive in general for duplicate content – you’re just not getting the most benefit. You’re website content isn’t being what it could be – a contender.
But this should be common sense. Google wants and rewards original content. Google doesn’t like duplicate content, and it’s a footprint of most spam sites. You don’t want to look anything like a spam site.
The more you can make it look a human built every page on a page by page basis with content that doesn’t appear exactly in other areas of the site – the more Google will like it. Google does not like automation when it comes to building a website, that’s for clear. (Unique titles, meta descriptions, keyword tags, content.)
I don’t mind Category duplicate content – as with WordPress – it can even help sometimes to spread PR and theme a site. But I generally wouldn’t have tags and categories, for instance.
I’m not that bothered with ‘themeing’ at this point to recommend silo’ing your content or no-indexing your categories. If I am not theming enough with proper content and mini-silo’ing to related pages from this page and to this page I should go home. Most sites in my opinion don’t need to silo their content – the scope of the content is just not that broad.
Keep in mind Google won’t thank you for spidering a calendar folder with 10,000 blank pages on it – why would they. They may even algorythmically tick you off.
PS – Duplicate content found on other sites? Now that’s a totally diferent problem.
UPDATED: See Google Advice on Duplicate Content.