Saving custom fields during Post Save

Then I have the following function to save my data:

function myplugin_save_postdata ( $post_id ){
if (defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
    return;

if (!wp_verify_nonce( $_POST['myplugin_noncename'], plugin_basename ( __FILE__ ) ) )
    return;

if ( 'page' == $_POST['post_type'] ){
    if (!current_user_can( 'edit_page', $post_id ) )
        return;
}else{
    if (!current_user_can( 'edit_post', $post_id ) )
        return;
}

$address1 = mysql_real_escape_string($_POST['address1']);
$address2 = mysql_real_escape_string($_POST['address2']);
$city = mysql_real_escape_string($_POST['city']);
$state = mysql_real_escape_string($_POST['state']);
$zip = mysql_real_escape_string($_POST['zip']);

$sql = "Insert into custom_vendor_info (post_id, address1, address2, city, state, zip) values('$post_id', '$address1', '$address2', '$city', '$state', '$zip')";
$_SESSION['my_admin_notices'] .= '<div class="updated"><p>'.$sql.'</p></div>';
$reuslt = mysql_query($sql);
return true;
}

However, I’m getting two entries into my custom table. I need to get the post id that WordPress saves originally so I can have a link between my custom table and the wp_posts table. How can I do this? Thanks in advance!