A canonoical tag tells a search engine where the ‘master’ version of this page is. The
href element of the tag points to a URL within the same (or different) website which is to be treated as the main page. Often content management systems will set up a canonical tag on every single page by default, pointing back to itself. This is no issue at all, however where you have duplicate or identical content across multiple pages, you should be using the canonical tag to point all of the duplicates to the main page.
When crawling all of your pages, if most of the content is duplicate across them, they may miss unique content. Equally, having identical content across multiple URLs will almost definitely damage the ranking ability of each page – essentially they’ll be competing with each other which can lead to none of your pages ranking well if picked up by the search engine. Finally, if you do manage to get a page ranking with duplicate content, it might not be the ‘master’ of these identical pages that is ranking well.
The answer is simple. Imagine you have an eCommerce store selling t-shirts. Each size or colour of this t-shirt will be a unique URL and different page, but you’re not likely have different descriptions against each one. If they’re all the same, and the price is the same, the only difference is one attribute (colour or size). This means search engines will treat this as duplicate content, so this is an example of where you’d use a canonical tag to point each size and colour at the best seller.
If you are to use canonical tags, make sure to set them all up correctly and don’t create any mixed signals. For example, having page one canonicalising to page two, and page three canonicalising back to page one would be detrimental – make sure all canonical tags across duplicate pages point to one of your pages, and it’s this one page that you want to rank better than the others.
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