You should add a canonical tag 'before the </HEAD> tag' in the custom code 'before the </HEAD> tag' on each page in the properties with the following syntax:

<link rel="canonical" href="https://www.mywebsite.com/page.html" />

This prevents duplicate content.

You have to replace www.mywebsite.com with the real domain. And you have to replace page.html with the respective page name + extension.

You have to make sure that pages that may actually and intentionally have the same or similar content may only have a canonical tag on one page. For example, if you have two pages that have the same content, the canonical tag from both pages can only point to one of the pages.

1. http://www.index.html is not a valid domain because it ends with the html extension. As I wrote above, a valid domain is http://www.mywebsite.com A valid page of this domain is http://www.mywebsite.com/index.html Here you have to enter <link rel="canonical" href="http://www.mywebsite.com/index.html" /> as described above.
2. Duplicate pages can be in a shop, for example, if you have a green t-shirt page and a yellow t-shirt page. The basic page content is the same, the only difference is the price. Google and other search engines do not know this and only recognize the same content. This means that the page is duplicated and is then devalued or even penalized.
3. For "cleanliness" reasons and to avoid penalties and deprecation of your website, it is a must to include canonical tags.

Ok, then you are really on the safe side.