what’s difference between canonical & duplicate pages URL for Google Indexing

This articles originally from https://support.google.com/webmasters/answer/139066

If you have a single page accessible by multiple URLs, or different pages with similar content (for example, a page with both a mobile and a desktop version), Google sees these as duplicate versions of the same page. Google will choose one URL as the canonical version and crawl that, and all other URLs will be considered duplicate URLs and crawled less often.

If you don’t explicitly tell Google which URL is canonical, Google will make the choice for you, or might consider them both of equal weight, which might lead to unwanted behavior, as explained below in Why should I choose a canonical URL?

This is probably more information about canonicalization than you need to know, so feel free to skip it. However, we’ll provide it in case you like this sort of thing.
When Googlebot indexes a site, it tries to determine the topics covered in each page. If Googlebot finds multiple pages on the same site that seem to be about the same thing, it chooses the page that it thinks is the most complete and useful, and marks it as canonical. The canonical page will be crawled most regularly; the duplicates are crawled less frequently in order to save crawling budget on your site. So if you don’t tell Googlebot which is the canonical page, and you decide that a different page is canonical, you might be spending energy updating a page that Googlebot won’t be indexing very often or displaying in search results.
Google uses the canonical pages on your site as the gold standard of your site’s content, as far as evaluating content and quality, and the Google Search result usually points to the canonical page, unless one of the duplicates is explicitly better suited to a user’s query: for example, the search result will probably point to the mobile page if the user is on a mobile device, even if the desktop page is marked as canonical.
Google chooses the canonical page based on a number of factors (or signals), such as whether the page is served via http or https; the user’s declared preferred domain; page quality; presence of the URL in a sitemap; and any “rel=canonical” labeling. You cannot force Google’s choice of canonical page, but you can influence the choice by using one or more of the techniques shown here. for more details, please visit here

 

Which URL does Google consider canonical (or duplicate)?

Use the URL Inspection tool to learn which page Google considers canonical. Note that even if you explicitly designate a canonical page, Google might choose a different canonical for various reasons, such as performance or content.