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If the canonical page disappears when it's out of stock, the user will get a 404 error when they try to access that URL. You'll need to canonicalize duplicates to a generic product or category page that won't disappear when the product is out of stock.  Furthermore, if a product goes out of stock and you'll have more in stock later, it's best (for SEO purposes) to leave the page with an "out of stock" notice rather than remove it completely from the website.