Sitemaps: CPT sitemaps cause duplicate sitemap files

### Steps to reproduce

1. Add a plugin that adds a CPT to your site.
2. Add many posts to that Post Type.
3. Whitelist that post type:
```php
/**
* Add Trakt events to sitemaps.
*/
function jeherve_add_trakt_sitemaps( $post_types ) {
$post_types[] = 'traktivity_event';
return $post_types;
}
add_filter( 'jetpack_sitemap_post_types', 'jeherve_add_trakt_sitemaps' );
```
4. Flush existing sitemaps and deactivate / reactivate the module to rebuild the sitemap.
5. When the sitemap gets rebuilt, you will see some of the sitemaps are duplicated. See some examples here:
https://jeremy.hu/sitemap-index-1.xml
https://jeremy.hu/image-sitemap-index-1.xml

Start by reproducing the issue with a custom post type, many posts, and the jetpack_sitemap_post_types filter, then trace the sitemap rebuild path. Compare the generated sitemap index and image sitemap index URLs after flushing and rebuilding. Done means whitelisted custom post types no longer produce duplicate sitemap files.

Written by the indexing model from the issue text.