TheOptimizationIdiot - 12:36 pm on Mar 6, 2013 (gmt 0)
This is definitely an interesting question :-). Before the rel=canonical link element was announced, using noindex robots meta tags was one way that webmasters were directing us towards canonicals, so this is certainly something we know and understand. However, with the coming of the rel=canonical link element, the optimal way of specifying a canonical is (apart from using a 301 redirect to the preferred URL) is to only use the rel=canonical link element.
One reason for this is that we sometimes find a non-canonical URL first. If this URL has a noindex robots meta tag, we might decide not to index anything until we crawl and index the canonical URL. Without the noindex robots meta tag (with the rel=canonical link element) we can start by indexing that URL and show it to users in search results.
I would still use both, because it has exactly the desired effect and the canonical is recognized on noindexed pages, according to JohnMu quoted above.
When Google detects duplicate content, such as variations caused by URL parameters, we group the duplicate URLs into one cluster and select what we think is the "best" URL to represent the cluster in search results. We then consolidate properties of the URLs in the cluster, such as link popularity, to the representative URL. Consolidating properties from duplicates into one representative URL often provides users with more accurate search results.
To improve this process, we recommend using the parameter handling tool to give Google information about how to handle URLs containing specific parameters. We'll do our best to take this information into account; however, there may be cases when the provided suggestions may do more harm than good for a site.
They'll do their best to take the ignore into account.
Hmmm I wonder why would I want "safeguards" (canonical & noindex) on the duplicate pages?