Realized that using both xmlsitemap_menu and xmlsitemap_node can result in duplicate items in the sitemap for nodes also having a link in a menu. After a quick search in the issue queue it seems that this bothered a few ppl and using other submodules can cause the same problem:
https://drupal.org/node/596008
https://drupal.org/node/631218
https://drupal.org/node/1999958
My proposed solution would be to add an option to always exclude duplicate entries from the sitemap. I'm waiting for some feedback/opinions before actually starting to work on a patch.