BuddyPress.org

one quick update:
the double add_action is actually taken from an example at bp-core-settings.php (in the bp_core_add_settings_nav() function) NOT bp-xprofile.php, which does in fact use bp_setup_nav to hook into for setting up profile nav/sub-nav items.

my guess is bp_core_new_subnav_item adds to an array, and includes a dupe check to the array (if !array_key_exists or some such) because the sub-nav items only show once…

ahh, hooks — a bit like magic!