A Google Webmaster Help thread has one SEO nervous that she has multiple sitemaps file but the files have duplication amongst the URLs listed in them.
So she has one sitemap file that may have the same and more URLs listed in them as another sitemap file. She asked if the redundancy is okay or if this can lead to issues.
Google's John Mueller said not only is it okay, it is "normal." John said he pinged the Sitemaps 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 about it, I know many sites that have multiple owners and authorized users of their Google Webmaster Tools. They may have the developer, design, webmaster, owner and then multiple SEO companies. Some of them may submit different sitemap files.
Duplication, is common and Google handles it in this case.
Forum discussion at Google Webmaster Help.