How to select single category in menu if post have two categories?

Yes, Finally i had done it. I done changes in nav-menu-template.php.

I created one variable and initialized with 1 above

foreach ( (array) $menu_items as $key => $parent_item ) {
this code.

and add conditional code

if($repetation == '1'){//to stop second selection of menu
     $classes[] = 'current-menu-parent';
        $menu_items[$key]->current_item_parent = true;
            $repetation++;
    }
         else{
    //echo $repetation;
         }

inbetween

if ( in_array( $parent_item->db_id, $active_parent_item_ids ) ) {

...}

this.

whole code look like

  $repetation = '1';
// set parent's class
foreach ( (array) $menu_items as $key => $parent_item ) {
    $classes = (array) $parent_item->classes;
    $menu_items[$key]->current_item_ancestor = false;
    $menu_items[$key]->current_item_parent = false;

    if (
        isset( $parent_item->type ) &&
        (
            // ancestral post object
            (
                'post_type' == $parent_item->type &&
                ! empty( $queried_object->post_type ) &&
                is_post_type_hierarchical( $queried_object->post_type ) &&
                in_array( $parent_item->object_id, $queried_object->ancestors ) &&
                $parent_item->object != $queried_object->ID
            ) ||

            // ancestral term
            (
                'taxonomy' == $parent_item->type &&
                isset( $possible_taxonomy_ancestors[ $parent_item->object ] ) &&
                in_array( $parent_item->object_id, $possible_taxonomy_ancestors[ $parent_item->object ] ) &&
                (
                    ! isset( $queried_object->term_id ) ||
                    $parent_item->object_id != $queried_object->term_id
                )
            )
        )
    ) {
        $classes[] = empty( $queried_object->taxonomy ) ? 'current-' . $queried_object->post_type . '-ancestor' : 'current-' . $queried_object->taxonomy . '-ancestor';
    }

    if ( in_array(  intval( $parent_item->db_id ), $active_ancestor_item_ids ) ) {
        $classes[] = 'current-menu-ancestor';
        $menu_items[$key]->current_item_ancestor = true;
    }
    if ( in_array( $parent_item->db_id, $active_parent_item_ids ) ) {

                if($repetation == '1'){//to stop second selection of menu
                    $classes[] = 'current-menu-parent';
        $menu_items[$key]->current_item_parent = true;
                    $repetation++;
                }
                else{
                    //echo $repetation;
                }

    }
    if ( in_array( $parent_item->object_id, $active_parent_object_ids ) )
        $classes[] = 'current-' . $active_object . '-parent';

    if ( 'post_type' == $parent_item->type && 'page' == $parent_item->object ) {
        // Back compat classes for pages to match wp_page_menu()
        if ( in_array('current-menu-parent', $classes) )
            $classes[] = 'current_page_parent';
        if ( in_array('current-menu-ancestor', $classes) )
            $classes[] = 'current_page_ancestor';
    }

    $menu_items[$key]->classes = array_unique( $classes );

}