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

My website's post have one, two, three etc categories depends on articles. My menu is made of categories. when i select post (open whole article) it shows post category in menu.

Problem : when i select such article who have two category that time menu shows two selection of menus. how can i show only one category (first category)?

In above image you can see that when i select some post who have two category that time two menu selected.

Shall do changes in function.php, nav-menu-template.php or anywhere else....

foreach ( (array) $menu_items as $key => $menu_item ) {

    $menu_items[$key]->current = false;

    $classes = (array) $menu_item->classes;
    $classes[] = 'menu-item';
    $classes[] = 'menu-item-type-' . $menu_item->type;
    $classes[] = 'menu-item-object-' . $menu_item->object;

    // if the menu item corresponds to a taxonomy term for the currently-queried non-hierarchical post object
    if ( $wp_query->is_singular && 'taxonomy' == $menu_item->type && in_array( $menu_item->object_id, $possible_object_parents ) ) {
        $active_parent_object_ids[] = (int) $menu_item->object_id;
        $active_parent_item_ids[] = (int) $menu_item->db_id;
        $active_object = $queried_object->post_type;

    // if the menu item corresponds to the currently-queried post or taxonomy object
    } elseif (
        $menu_item->object_id == $queried_object_id &&
        (
            ( ! empty( $home_page_id ) && 'post_type' == $menu_item->type && $wp_query->is_home && $home_page_id == $menu_item->object_id ) ||
            ( 'post_type' == $menu_item->type && $wp_query->is_singular ) ||
            ( 'taxonomy' == $menu_item->type && ( $wp_query->is_category || $wp_query->is_tag || $wp_query->is_tax ) && $queried_object->taxonomy == $menu_item->object )
        )
    ) {
        $classes[] = 'current-menu-item';
        $menu_items[$key]->current = true;
        $_anc_id = (int) $menu_item->db_id;

        while(
            ( $_anc_id = get_post_meta( $_anc_id, '_menu_item_menu_item_parent', true ) ) &&
            ! in_array( $_anc_id, $active_ancestor_item_ids )
        ) {
            $active_ancestor_item_ids[] = $_anc_id;
        }

        if ( 'post_type' == $menu_item->type && 'page' == $menu_item->object ) {
            // Back compat classes for pages to match wp_page_menu()
            $classes[] = 'page_item';
            $classes[] = 'page-item-' . $menu_item->object_id;
            $classes[] = 'current_page_item';
        }
        $active_parent_item_ids[] = (int) $menu_item->menu_item_parent;
        $active_parent_object_ids[] = (int) $menu_item->post_parent;
        $active_object = $menu_item->object;

    // if the menu item corresponds to the currently-requested URL
    } elseif ( 'custom' == $menu_item->object ) {
        $_root_relative_current = untrailingslashit( $_SERVER['REQUEST_URI'] );
        $current_url = set_url_scheme( 'http://' . $_SERVER['HTTP_HOST'] . $_root_relative_current );
        $raw_item_url = strpos( $menu_item->url, '#' ) ? substr( $menu_item->url, 0, strpos( $menu_item->url, '#' ) ) : $menu_item->url;
        $item_url = untrailingslashit( $raw_item_url );
        $_indexless_current = untrailingslashit( preg_replace( '/' . preg_quote( $wp_rewrite->index, '/' ) . '$/', '', $current_url ) );

        if ( $raw_item_url && in_array( $item_url, array( $current_url, $_indexless_current, $_root_relative_current ) ) ) {
            $classes[] = 'current-menu-item';
            $menu_items[$key]->current = true;
            $_anc_id = (int) $menu_item->db_id;

            while(
                ( $_anc_id = get_post_meta( $_anc_id, '_menu_item_menu_item_parent', true ) ) &&
                ! in_array( $_anc_id, $active_ancestor_item_ids )
            ) {
                $active_ancestor_item_ids[] = $_anc_id;
            }

            if ( in_array( home_url(), array( untrailingslashit( $current_url ), untrailingslashit( $_indexless_current ) ) ) ) {
                // Back compat for home link to match wp_page_menu()
                $classes[] = 'current_page_item';
            }
            $active_parent_item_ids[] = (int) $menu_item->menu_item_parent;
            $active_parent_object_ids[] = (int) $menu_item->post_parent;
            $active_object = $menu_item->object;

        // give front page item current-menu-item class when extra query arguments involved
        } elseif ( $item_url == $front_page_url && is_front_page() ) {
            //$classes[] = 'current-menu-item';
        }

        if ( untrailingslashit($item_url) == home_url() )
            $classes[] = 'menu-item-home';
    }

    // back-compat with wp_page_menu: add "current_page_parent" to static home page link for any non-page query
    if ( ! empty( $home_page_id ) && 'post_type' == $menu_item->type && empty( $wp_query->is_page ) && $home_page_id == $menu_item->object_id )
        $classes[] = 'current_page_parent';

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

`

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 );

}