public function getVatValidationUserMessage($customerAddress, $customerGroupAutoAssignDisabled, $validationResult)
{
    $message = '';
    $isError = true;
    $customerVatClass = $this->getCustomerVatClass($customerAddress->getCountryId(), $validationResult);
    $groupAutoAssignDisabled = Mage::getStoreConfigFlag(self::XML_PATH_CUSTOMER_VIV_GROUP_AUTO_ASSIGN);

    $willChargeTaxMessage    = $this->__('You will be charged tax.');
    $willNotChargeTaxMessage = $this->__('You will not be charged tax.');

    if ($validationResult->getIsValid()) {
        $message = $this->__('Your VAT ID was successfully validated.');
        $isError = false;

        if (!$groupAutoAssignDisabled && !$customerGroupAutoAssignDisabled) {
            $message .= ' ' . ($customerVatClass == self::VAT_CLASS_DOMESTIC
                ? $willChargeTaxMessage
                : $willNotChargeTaxMessage);
        }
    } else if ($validationResult->getRequestSuccess()) {
        $message = sprintf(
            $this->__('The VAT ID entered (%s) is not a valid VAT ID.') . ' ',
            $this->escapeHtml($customerAddress->getVatId())
        );
        if (!$groupAutoAssignDisabled && !$customerGroupAutoAssignDisabled) {
            $message .= $willChargeTaxMessage;
        }
    }
    else {
        $contactUsMessage = sprintf($this->__('If you believe this is an error, please contact us at %s'),
            Mage::getStoreConfig(self::XML_PATH_SUPPORT_EMAIL));

        $message = $this->__('Your Tax ID cannot be validated.') . ' '
            . (!$groupAutoAssignDisabled && !$customerGroupAutoAssignDisabled
                ? $willChargeTaxMessage . ' ' : '')
            . $contactUsMessage;
    }

    $validationMessageEnvelope = new Varien_Object();
    $validationMessageEnvelope->setMessage($message);
    $validationMessageEnvelope->setIsError($isError);

    return $validationMessageEnvelope;
}

if (!$validationMessage->getIsError()) {
    Mage::getSingleton('customer/session')->addSuccess($validationMessage->getMessage());
} else {
    Mage::getSingleton('customer/session')->addError($validationMessage->getMessage());
}

<block type="core/messages" name="global_messages" as="global_messages"/>
<block type="core/messages" name="messages" as="messages"/>

/*
 * @author: ..
 * This class overrides the addMessage for customer.
 * Because when BTW is validated, a message appears multiple times.
 * With this rewrite we check if the message is not already added.
 */
class MYMODULE_Customer_Model_Session extends Mage_Customer_Model_Session
{
    /*
     * Adding new message to message collection
     *
     * @param   Mage_Core_Model_Message_Abstract $message
     * @return  Mage_Core_Model_Session_Abstract
     */
    public function addMessage(Mage_Core_Model_Message_Abstract $message)
    {
        // get all current messages
        $sessionMessages = $this->getMessages()->getItems();
        // loop thrue messages and find message with same text
        $found = false;
        foreach($sessionMessages as $sessionMessage) {
            if ( $sessionMessage->getCode() == $message->getCode() ) {
                $found = true; // found message with same text
            }
        }
        if ( !$found ) { // only add message if not already in session
            $this->getMessages()->add($message);
            Mage::dispatchEvent('core_session_abstract_add_message');
        }
        return $this;
    }
}