Dynamic E-commerce websites (like Os commerce, etc) are plague with duplicate content. This month, search engine communities agreed to release the “link rel=”canonical” tag. The main objective is to specify your canonical URL among several other URLs that are duplicates with the canonical URL. Long before, we have used techniques like robots.txt, 301 redirection and putting meta no index, it works; but only 301 redirection transfers link juices, page rank and other URL properties that could ultimately help rankings.

The downside of 301 redirection is that it is often difficult to do. Link rel=”canonical” tag is very easy to perform, you will just copy paste the code to the template and its done. There are some very good FAQ on this, particularly on the Google official link rel=”canonical” tag page.

I have written down some advanced technical questions pertaining to the implementation of this tag with supporting evidences:

First question:) Do we need to place link rel=canonical tag to the canonical URLs also, aside from the duplicate URLs?

Answer: Though its not obligatory, its fine to add this tag to canonical page.

Quote from googlewebmastercentral.blogspot.com:

Yes, it’s absolutely okay to have a self-referential rel=”canonical”. It won’t harm the system and additionally, by including a self-reference you better ensure that your mirrors have a Rel=”canonical” to you.

It’s not obligatory though to add the tag to canonical page.
Quote from Matt Cutts: (http://www.mattcutts.com/blog/canonical-link-tag/)
1. On the true canonical page, you can just leave out the canonical tag
2. On the true canonical page, you can keep the canonical tag, but make the tag point back to the same url.

Second question:) Is it possible for meta no index to be replaced with link rel canonical to pass link juices to the homepage? (this will divert more link juices to the homepage, helping it to rank)

Scenario #1: In all pages in the domain that are placed with meta no-index, and these pages are not exactly the same but similar, does link rel=”canonical” appropriate?
Answer: Google only recommends placing canonical tag to highly similar pages only.

Quote from googlewebmastercentral.blogspot.com:

We allow slight differences, e.g., in the sort order of a table of products. We also recognize that we may crawl the canonical and the duplicate pages at different points in time, so we may occasionally see different versions of your content. All of that is okay with us.

Quote from Matt Cutts:

Q: Do the pages have to be bit-for-bit identical?

A: No, but they should be similar. Slight differences are okay.

Scenario #2: How about using it in unique pages but low valued pages (which are placed with noindex) then putting link rel=”canonical” to transfer leaking link juices to the homepage for example?

Answer: Pointing the canonical tag to unique pages in order to conserve the site’s links juices would likely be considered as deceptive, since this tag is used other than its original intended purpose.

Quote from Matt Cutts:

If we see abuse, we reserve the right to react as needed .

Good luck in your implementation with this tag.

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