This functions check before encoding / decoding if the string is already encoded / decoded.

Utf8Helper.php

<?php

/**
 * utf8_encode only once
 *
 * @param string $data The data
 * @return string The converted string
 */
function doUtf8($data)
{
    return !preg_match('!!u', $data) ? utf8_encode($data) : $data;
}

/**
 * utf8_decode only once
 *
 * @param string $data The data
 * @return string The converted string
 */
function undoUtf8($data)
{
    return preg_match('!!u', $data) ? utf8_decode($data) : $data;
}