Often website owners don’t even realise that they have duplicate content, which is understandable if they’ve never been made aware of what causes it to happen.
Varied URL's for the same page
Duplicate content is created when there are more than one URLs that lead to the same page. Google considers case sensitivity in URLs which is why the general rule of thumb is to have all your URLs short and lower case. Often times, people aren't aware of the content duplication aspect of SEO and this is why they don't keep their URLs in one case format. It also applied for analytical and URL tracking, for example if using UTM tags for PPC campaign tracking to a website, the different parameters and the order in which those parameters appear in the URL all account for duplications of the same page.
HTTP vs. HTTPs protocol
Similarly to how Google considers URLs case sensitive, it also considers secure and non secure connections as-well as testing and staging versions of your sites so long as the content is the exact same. In some cases, for the users with websites built in CMS (content management systems) such as Wordpress and CraftCMS, the CMS can create duplicate pages.
In the world of ecommerce, this is usually the number one cause for duplicated content. For example, in the product description sections of the website, manufacturers usually copy and paste, which is not good from an SEO standpoint or generally for your business. Business owners aren't aware that content such as this is scraped from similar sites and it's not actually original.