An interesting Google Webmaster Help thread discusses the old and new way of handling duplicate content. In the old days, you either 301 redirected page A to page B, when there was duplicate content. But if that wasn’t possible, you instructed the search engine not to index the duplicate page, by adding a noindex tag to the duplicate page. Some people had this automated through their CMS.

With the addition of the new rel=canonical link attribute, things have changed. The new attribute is basically a 301 redirect, without doing a real redirect. It tells the search engine, page B is a duplicate to page A. At the same time, this is hidden from the normal user of your site.

The Google Webmaster Help thread asked, can you use both, the noindex and rel=canonical tag on the same page?

Googler, JohnMu answers the question from Google’s perspective:

Bottom line, don’t do it, try to either not have duplicate content, or use a 301 redirect or use the rel=canonical. You can now stay away from using the noindex tag for these purposes.

Forum discussion at Google Webmaster Help.