You might be thinking, “Why would anyone duplicate a page?” and wrongly assume that canonicalization isn’t something you have to worry about. The problem is that we, as humans, tend to think of a page as a concept, such as your homepage. For search engines, though, every unique URL is a separate page. To a human, all of these URLs represent a single page. To a search crawler, though, every single one of these URLs is a unique “page.” Even in this limited example, we can see there are five copies of the homepage in play. In reality, though, this is just a small sample of the variations you might encounter.
Modern content management systems (CMS) and dynamic, code-driven websites exacerbate the problem even more. Many sites automatically add tags, allow multiple paths (and URLs) to the same content, and add URL parameters for searches, sorts, currency options, etc. You may have thousands of duplicate URLs on your site and not even realize it.