For Wordpress, add a Duplicate option to Posts & Pages without using a plugin.

SimplePostDuplicator.php

// SimplePostDuplicator - Adds a Duplicate option to Posts & Pages without using a plugin! https://snipsnip.pro/s/701

if (!class_exists('SimplePostDuplicator')) {
    class SimplePostDuplicator {
        public function __construct() {
            add_action('admin_action_simple_post_duplicator_duplicate_post_as_draft', array($this, 'duplicatePostAsDraft'));
            add_filter('post_row_actions', array($this, 'addDuplicatePostLink'), 10, 2);
            add_filter('page_row_actions', array($this, 'addDuplicatePostLink'), 10, 2);
        }

        /**
         * Function for post duplication. Duplicates appear as drafts. Users are redirected to the edit screen.
         */
        public function duplicatePostAsDraft() {
            if (!(isset($_GET['post']) || isset($_POST['post']) || (isset($_REQUEST['action']) && 'simple_post_duplicator_duplicate_post_as_draft' === $_REQUEST['action']))) {
                wp_die('No post to duplicate has been supplied!');
            }

            // Nonce verification
            if (!isset($_GET['duplicate_nonce']) || !wp_verify_nonce($_GET['duplicate_nonce'], basename(__FILE__))) {
                wp_die('Invalid security token.');
            }

            // Get the original post ID
            $post_id = (isset($_GET['post'])) ? absint($_GET['post']) : absint($_POST['post']);

            // Get the original post data
            $post = get_post($post_id);

            // Check if the user has permission to duplicate posts
            if (!current_user_can('edit_posts') || !$post) {
                wp_die('You do not have permission to duplicate posts.');
            }

            // Prepare the new post data
            $args = array(
                'comment_status' => $post->comment_status,
                'ping_status' => $post->ping_status,
                'post_author' => get_current_user_id(),
                '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 new post
            $new_post_id = wp_insert_post($args);

            if ($new_post_id) {
                // Duplicate the taxonomy terms
                $taxonomies = get_object_taxonomies($post->post_type);
                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 the post meta
                global $wpdb;
                $post_meta_infos = $wpdb->get_results($wpdb->prepare("SELECT meta_key, meta_value FROM $wpdb->postmeta WHERE post_id = %d", $post_id));
                foreach ($post_meta_infos as $meta_info) {
                    $meta_key = $meta_info->meta_key;
                    if ($meta_key === '_wp_old_slug') {
                        continue;
                    }
                    $meta_value = $meta_info->meta_value;
                    update_post_meta($new_post_id, $meta_key, $meta_value);
                }

                // 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 duplicate the original post.');
            }
        }

        /**
         * Add the duplicate link to the action list for post_row_actions
         */
        public function addDuplicatePostLink($actions, $post) {
            if (current_user_can('edit_posts')) {
                $duplicate_url = wp_nonce_url(admin_url('admin.php?action=simple_post_duplicator_duplicate_post_as_draft&post=' . $post->ID), basename(__FILE__), 'duplicate_nonce');
                $actions['duplicate'] = '<a href="' . esc_url($duplicate_url) . '" title="Duplicate this item" rel="permalink">Duplicate</a>';
            }
            return $actions;
        }
    }

    // Instantiate the class
    new SimplePostDuplicator();
}