Meta Box Only in CPT Edit Page, Not in Add New

Check the action of the current screen object for add:

function wpd_add_meta_box() {
    $screen = get_current_screen();
    if( 'add' != $screen->action ){
        add_meta_box(
            'myplugin_sectionid',
            'My meta box',
            'myplugin_meta_box_callback',
            'leads'
        );
    }
}
add_action( 'add_meta_boxes', 'wpd_add_meta_box' );