Insert post from frontend, check for duplicates in wpdb

<?php
/**
 * Template Name: Post Insert
 *
 * A custom page template that allows users to submit a post from the front end.
 * Based on the Twenty Ten page template.
 * The "Template Name:" bit above allows this to be selectable
 * from a dropdown menu on the edit page screen.
 *
 * @author iCosmin (http://icosmin.com)
 * @package WordPress
 * @subpackage Twenty_Ten
 * @since Twenty Ten 1.0
 */

get_header(); ?>

        <div id="container" class="one-column">
            <div id="content" role="main">

            <?php

                global $wpdb;
                global $post;
                $title = $_POST['title']; // get the inputted title
                $content = $_POST['content']; // get the inputted content
                $categorie = $_POST['cat'];  // get the category selected by user
                $zombies = $_POST['zombies'];
                # run a query to check for a post containing the data that our user is about to submit
                # store results in $verifica
                $sql = "
                SELECT wposts.*
                FROM $wpdb->posts wposts
                    LEFT JOIN $wpdb->postmeta wpostmeta ON wposts.ID = wpostmeta.post_id
                    LEFT JOIN $wpdb->term_relationships ON (wposts.ID = $wpdb->term_relationships.object_id)
                    LEFT JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)
                WHERE wposts.post_status = 'publish'
                    AND wposts.post_title = '$title'
                    AND wposts.post_content = '$content'
                    AND wpostmeta.meta_key = 'Zombies'
                    AND wpostmeta.meta_value = '$zombies'
                    AND $wpdb->term_taxonomy.taxonomy = 'category'
                    AND $wpdb->term_taxonomy.term_id IN($categorie)";
                $verifica = $wpdb->get_results($sql);

                  if( 'POST' == $_SERVER['REQUEST_METHOD'] ) { // if form has been submitted

                  # some validation
                      if(empty($title)) {
                        echo "Please give your post a title<br />";
                      }
                      if (empty($content)){
                        echo "Please write your post's content<br />";
                      }
                      if ($categorie == -1){
                           echo "Please assign a category to your post.<br />";
                      }
                      if (empty($zombies)) {
                            echo "Please input any number of Zombies";
                      }
                      # if there are no similar posts ($verifica is empty) and user filled in the fields, insert post
                      # also, redirect to the homepage to make sure we don't get 404-ed
                      if (empty($verifica) && !empty($title) && !empty($content) && $categorie != -1) {
                        $my_post = array(
                         'post_title' => $title,
                         'post_content' => $content,
                         'post_status' => 'draft',
                         'post_author' => 1,
                         'post_category' => array($categorie),
                          );

                    $my_post = wp_insert_post($my_post);
                    add_post_meta($my_post, 'Zombies', $zombies);
                    wp_redirect( home_url() );
                      }

                      # if $verifica is not empty, then we don't insert the post and we display a message
                      else if( !empty($verifica) ) { echo "You are trying to submit the same post twice! Be nice.";  }
                  }
            ?>
            <hr>
            <form action="" method="post" name="myForm">
                <p><label for="title">Title</label>
                <input type="text" name="title" id="title">
                </p>
                <p>
                <?php wp_dropdown_categories( 'show_option_none=Please choose a category&tab_index=4&taxonomy=category&hide_empty=0' ); ?>
                </p>
                <p>
                <label for="zombies">Zombies <small>(number; adds a custom field named 'Zombies' with the value you specify)</small></label>
                <input type="text" name="zombies" id="zombies">
                </p>
                <p>
                <label for="content">Post Content</label>
                <textarea name="content" id="content" cols="30" rows="10"></textarea>
                </p>
                <input type="submit" value="Insert post">
<?php wp_nonce_field( 'myForm' ); ?>

            </form>

            <hr>

            </div><!-- #content -->
        </div><!-- #container -->

<?php get_footer(); ?>