There are many reasons why instances of duplicate content may occur. Some of these reasons include:

Unaware Content Creators:

Sometimes, content creators don't know that putting the same content in different parts of a website can cause issues. For example, a blog post could mistakenly show up under two categories, like "Nutrition" and "Health Tips," which makes it appear twice with different web addresses.

Content Theft:

Other times, some bad-acting websites might steal your content and post it as their own. This creates a situation where the same content is found on different websites.

URL Variations:

Content might be reachable through several different web addresses due to small differences in the URLs. This can make search engines think they are different pages when they are not.

Tracking Codes:

When you use special codes in URLs to track where visitors come from, it can accidentally create duplicates. For example, the same page might show up with and without a tracking tag at the end of the web address.

Sorting and Filtering:

On online stores, when you sort or filter products, it can create unique URLs for the same set of products, which search engines might see as duplicate content.

Different Protocols and Prefixes:

If a website hasn't been set up properly, both secure (https) and non-secure (http) versions of a page could show up. The same goes for addresses with "www" and those without.

Test Servers:

Sometimes the test versions of web pages end up being found by search engines, creating duplicates of the live pages.

Multiple Homepage Links:

Homepages can sometimes be reached through several different addresses, which can be confusing for search engines.

Case Sensitivity:

Some websites treat upper and lower case in URLs differently, which can result in what seems like two different pages to a search engine.

Print and Mobile Versions:

Websites might have special versions of pages for printing or mobile users, which can accidentally create duplicates.

International Versions:

Websites that have versions for different countries might end up with duplicates if the same content is on multiple country-specific pages.

Tag and Category Overlap:

When articles or posts are tagged or put into categories, it can create multiple similar pages on a website.

Comment Sections:

If a page with comments is split across several URLs to make it easier to read, each one can be seen as a separate page by search engines.

Product Options:

For online shops, different colors or sizes of the same product can have their own URLs, leading to duplicates.