register_my_menus() redeclared causing fatal error

[admin]

I am creating a new wordpress theme and for menus using:

function register_my_menus() {
    register_nav_menus(
        array( 'header-menu' => __( 'Header Menu' ) )
    );
}
add_action( 'init', 'register_my_menus' );

This generates a fatal error:

Fatal error: Cannot redeclare register_my_menus() (previously declared in C:\wamp\www\wordpress\wp-content\themes\myTheme\functions.php:5) in C:\wamp\www\wordpress\wp-content\themes\myTheme\functions.php on line 9

This doesn't get called/used anywhere else.

My understanding is that the function is called

register_my_menus()

and then the menu is added initialized with the add_function.

Any ideas?