What are XML sitemaps?
You want search engines to crawl every important page of your website. But it can happen pages don’t have any internal links to them, which will make them hard to find. You can use an XML sitemap to make sure search engines can find and crawl all pages you deem essential on your website. An XML sitemap contains all important pages of a site to help search engines determine the structure of it.
XML sitemap module automatically generates sitemaps of your website which adhere to the new Google standard regarding multilingual content by creating hreflang sitemaps and image sitemaps. This generated sitemaps would be submitted to the search engines at regular intervals of time that you can set.
It is not, however, a guarantee that those URLs will be crawled or indexed. Search engines use information from Sitemaps to augment their usual crawl and discovery processes. A Sitemap does not affect the actual ranking of your pages. However, if it helps get more of your site crawled (by notifying search engines of URLs they didn't previously know about, and/or by helping search engines prioritize the URLs on your site), that can lead to increased presence and visibility of your site in their index.
What happens when multiple sites are sending the same URLs?
This is apparently normal if you submit Sitemap files with the same URLs listed multiple times. Since submitting the same URLs multiple times doesn't change anything with regards to your site's crawling & indexing, I'd just submit it once if you want the correct counts.