This is what the quote says:
Having duplicate content can affect your site in a variety of ways; but unless you’ve been duplicating deliberately, it’s unlikely that one of those ways will be a penalty. This means that:
* You typically don’t need to submit a reconsideration request when you’re cleaning up innocently duplicated content.
* If you’re a webmaster of beginner-to-intermediate savviness, you probably don’t need to put too much energy into worrying about duplicate content, since most search engines have ways of handling it.
* You can help your fellow webmasters by not perpetuating the myth of duplicate content penalties! The remedies for duplicate content are entirely within your control. Here are some good places to start.
We (SEO professionals) do understand that in this case there is no penalty for having duplicate content.
The problem is: Google doesn’t have to enforce any penalty, duplicate content is bad for you by default.
It’s actually pretty logical, let me explain:
You have a product page that can be reached via two different categories, which would create two different URLs with identical content:
Your store is sending the link juice (the PageRank value) to two of these URLs. If you only had one instead, it would receive double the amount of link juice.
Now imagine what happens if you have multiple products in multiple categories all over your store. How much link juice are you actually throwing away? A lot.
Different URLs when product is listed in multiple categories is just one of the cases in which Magento serves almost identical content on two different URLs. This is why Magento is not really on-site SEO perfect out of the box and you need someone to “make it right”.
In the recent release of Magento 220.127.116.11, there have been some on-site SEO improvements with the default theme, so I can see Magento team is working hard on getting Magento on-site SEO perfect.
Please note that Magento is not “bad” or “terrible” in on-site SEO, as the matter of fact, most of the other open source e-commerce solutions have much bigger SEO problems out of the box. We’re just trying to reach perfection here, and that’s why we advise you to solve the duplicate content problems at your Magento store.