double variable

hey
looks like i'm getting two variables stored by content type:
- [content_type]_menu_settings
- menu_[content_type]

example:
- book_menu_settings

a:6:{
s:18:"menu-footer-menu-1";s:18:"menu-footer-menu-1";
s:18:"menu-footer-menu-2";s:18:"menu-footer-menu-2";
s:18:"menu-footer-menu-3";s:18:"menu-footer-menu-3";
s:10:"navigation";s:10:"navigation";
s:13:"primary-links";s:13:"primary-links";
s:15:"secondary-links";s:15:"secondary-links";
}

- menu_book

a:6:{
i:0;s:18:"menu-footer-menu-1";
i:1;s:18:"menu-footer-menu-2";
i:2;s:18:"menu-footer-menu-3";
i:3;s:10:"navigation";
i:4;s:13:"primary-links";
i:5;s:15:"secondary-links";
}

this normal? is a second variable really needed?