/wp-content/plugins/project-extra/includes/helpers/duplicate-post.php

<?php
/*
 * Crear duplicado de las entradas como borrador y redirige luego a la pantalla de edición
 */
function rd_duplicate_post_as_draft(){
    global $wpdb;
    if (! ( isset( $_GET['post']) || isset( $_POST['post']) || ( isset($_REQUEST['action']) && 'rd_duplicate_post_as_draft' == $_REQUEST['action'] ) ) ) {
        wp_die('No post to duplicate has been supplied!');
    }

    /*
    * get the original post id
    */
    $post_id = (isset($_GET['post']) ? $_GET['post'] : $_POST['post']);
    /*
    * and all the original post data then
    */
    $post = get_post( $post_id );

    /*
    * if you don't want current user to be the new post author,
    * then change next couple of lines to this: $new_post_author = $post->post_author;
    */
    $current_user = wp_get_current_user();
    $new_post_author = $current_user->ID;

    /*
    * if post data exists, create the post duplicate
    */
    if (isset( $post ) && $post != null) {

        /*
        * new post data array
        */
        $args = array(
            'comment_status' => $post->comment_status,
            'ping_status' => $post->ping_status,
            'post_author' => $new_post_author,
            'post_content' => $post->post_content,
            'post_excerpt' => $post->post_excerpt,
            'post_name' => $post->post_name,
            'post_parent' => $post->post_parent,
            'post_password' => $post->post_password,
            'post_status' => 'draft',
            'post_title' => $post->post_title,
            'post_type' => $post->post_type,
            'to_ping' => $post->to_ping,
            'menu_order' => $post->menu_order
        );

        /*
        * insert the post by wp_insert_post() function
        */
        $new_post_id = wp_insert_post( $args );

        /*
        * get all current post terms ad set them to the new post draft
        */
        $taxonomies = get_object_taxonomies($post->post_type); // returns array of taxonomy names for post type, ex array("category", "post_tag");
        foreach ($taxonomies as $taxonomy) {
            $post_terms = wp_get_object_terms($post_id, $taxonomy, array('fields' => 'slugs'));
            wp_set_object_terms($new_post_id, $post_terms, $taxonomy, false);
        }

        /*
        * duplicate all post meta
        */
        $post_meta_infos = $wpdb->get_results("SELECT meta_key, meta_value FROM $wpdb->postmeta WHERE post_id=$post_id");
        if (count($post_meta_infos)!=0) {
            $sql_query = "INSERT INTO $wpdb->postmeta (post_id, meta_key, meta_value) ";
            foreach ($post_meta_infos as $meta_info) {
                $meta_key = $meta_info->meta_key;
                $meta_value = addslashes($meta_info->meta_value);
                $sql_query_sel[]= "SELECT $new_post_id, '$meta_key', '$meta_value'";
            }
            $sql_query.= implode(" UNION ALL ", $sql_query_sel);
            $wpdb->query($sql_query);
        }

        /*
        * finally, redirect to the edit post screen for the new draft
        */
        wp_redirect( admin_url( 'post.php?action=edit&post=' . $new_post_id ) );
        exit;
    } else {
        wp_die('Post creation failed, could not find original post: ' . $post_id);
    }
}
add_action( 'admin_action_rd_duplicate_post_as_draft', 'rd_duplicate_post_as_draft' );

/*
 * Add the duplicate link to action list for post_row_actions
 */
function rd_duplicate_post_link( $actions, $post ) {
    if (current_user_can('edit_posts')) {
        $actions['duplicate'] = '<a href="admin.php?action=rd_duplicate_post_as_draft&amp;post=' . $post->ID . '" title="Duplicar este elemento" rel="permalink">Duplicar</a>';
    }
    return $actions;
}

add_filter( 'post_row_actions', 'rd_duplicate_post_link', 10, 2); /* for posts */
add_filter( 'page_row_actions', 'rd_duplicate_post_link', 10, 2); /* for pages */