From my experience the requirement is usually opposite - achieving same slugs when WP stubbornly refuses to generate them. :)
In very few words slug might overlap between hierarchical and non-hierarchical post types, but not within them separately. Since in your case post (non-hierarchical) and page (hierarchical) are involved, WP happily allows slug reuse.
If you want to manually force WP to de-duplicate page's slug your best option is probably to force duplication check to fail and trigger numeric suffix logic. From the look at
wp_unique_post_slug() source that can be done via following filter:
apply_filters( 'wp_unique_post_slug_is_bad_hierarchical_slug', false, $slug, $post_type, $post_parent )