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Unfortunately, I don't know Etsy well, but I think Thomas is right on track - you don't necessarily want to pre-emptively add them, especially if you're not clear on how they work. Typically, it's more for preventing duplicate content issues. We're re-evaluating how we grade the canonical tag, since this is a bit confusing. It's a good practice on some pages (like your home-page) that naturally tend to have more than one version, but adding it sitewide can be tricky.