Duplication In Sitemap Files Is Fine With Google
That user has one sitemap file that she has the similar and more URLs listed in them as another sitemap file. She asked if the repetition is fine or if this can lead to issues.
After that John Mueller said that not only is it okay, it is "normal." John said he knocked the Sitemap team who said:
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 (alternately, if the count doesn't bother you, you can also just leave it like this.
If you think over it, I know various websites have several owners and authorized users of their Google Webmaster Tools account. I think they have the web developer, creative designer webmaster, owner and then several SEO companies. Some of them may submit different sitemap files.
Duplication is general and Google deals it in this case.