A Canonical Issue (also known as duplicate content) is when different versions of a website: 1) http://yoursite.com, 2) https://yoursite.com, 3) http://www.yoursite.com, 4) https://www.yoursite.com are seen as duplicates by the search engines. To avoid that, you can either add a noindex meta-tag in the non-canonical copies and additionally 301 redirects to the canon.