Duplicate content has become a bigger problem for many small business owners and webmasters in the last couple of years. The main reason why is because the Google Panda and Penguin updates targeted specific website issues that in effect penalize duplicate content. And one of the biggest culprits of this duplicate content issue is that webmasters set their WordPress or other CMS settings to allow Google to crawl and index tag and category pages. Don’t do that.
What happens is you write a blog post, categorize it in one or two categories, then give it five to ten tags. Google crawls the blog post, each category, and each tag. Then it starts indexing them.
If you have your settings set so that category and tag pages display a summary only, then it can initially appear like different content. Google crawls and indexes the pages. Then, on a later recrawl, they discover the duplicate content and your entire site is de-listed.
All of this can be avoided by simply not allowing Google to crawl or index your tag and category pages at all. What you want is for your blog posts and pages to be indexed. The tag and category pages are there for the benefit of your human readers, to allow them an easy way to find information they may be looking for.
Nip your duplicate content issues in the bud by getting your CMS settings right in the first place.