SEO: Canonical Tag for Duplicate Content

Problem: Our redux and old blog posts are essentially considered duplicate content by search engine crawlers, and although it will not hurt us as a whole, they will compete with each other for certain keywords – which we definitely don't want.

Proposed Solution: We need to tell Google which of the two posts we want indexed on Google when they search for them. It makes sense to use the newer posts, however, keep in mind that the older posts are the ones that are already ranking for certain keywords. Using a canonical tag is a good way to tell Google that one of the posts is almost like a redirect to the other post, and it will use that one to index instead. According to Google, canonical tags are treated just like 301 redirects, which allows them to retain their SEO benefits. There will be a very, very slight impact, but Matt Cutts says that this is not even noticeable.

You can read more about this here: http://www.hobo-web.co.uk/duplicate-content-problems/