Magento 1.6 – How to Check if Customer is Subscribed to Newsletter

Goto app/code/core/Mage/Page/Block/Template/Links.php

copy to app/code/local/Mage/Page/Block/Template/Links.php then goto addLink() function goto

$link = new Varien_Object(array(

add below code

if($label=='My Account'):
         $loggedIn = Mage::getSingleton('customer/session')->isLoggedIn();
         if( $loggedIn){
            $label=Mage::getSingleton('customer/session')->getCustomer()->getName();
         }
        endif;

and My Account depend on xml code <label>My Account</label>

Try this

Step1: trigger an event on basic of customer_save_before

<global>
    <events>
        <customer_save_before>
            <observers>
                <stockalert>
                    <type>singleton</type>
                    <class>magento37890/observer</class>
                    <method>checkDisplay</method>
                </stockalert>
            </observers>
        </customer_save_before>
    </events>
</global>       

Step2:

In before event give you can get customer id if customer is old,then checking current value and past value match or not. If not match then checking this value exit in Rest of collection. If exit then send a flag (false) and

throw error   using throw Mage::exception
which prevent data save

.....

if($flagofmy==false){

                throw Mage::exception(
                    'Mage_Customer', Mage::helper('customer')->__('This customer lastname already exists'),
                    Mage_Customer_Model_Customer::EXCEPTION_EMAIL_EXISTS
                );
}

full Logic

<?php
class Stackexchange_Magento37890_Model_Observer
{
public function checkDisplay(Varien_Event_Observer $observer){
    $flagofmy=true;
    //eeeeee
    $customer=$observer->getEvent()->getCustomer();
    Mage::log('My log entry'.$customer->getId(), null, 'Magento37890.log');
    /* if customer  is old customer */
    if($customer->getId()){
            if($customer->getOrigData('lastname')!=$customer->getData('lastname')){

            Mage::log($customer->getData('lastname').'My log entry'.$customer->getId(), null, 'Magento37890.log');

            $flagofmy=$this->HasLastName($customer->getData('lastname'));
            if($flagofmy==false){

                throw Mage::exception(
                    'Mage_Customer', Mage::helper('customer')->__('This customer lastname already exists'),
                    Mage_Customer_Model_Customer::EXCEPTION_EMAIL_EXISTS
                );

                }
            }else{
                    Mage::log($customer->getData('lastname').' Step A _1'.$customer->getId(), null, 'Magento37890.log');

            }

    }else{
        /* New Customer */

            $flagofmy=$this->NewHasLastName($customer->getData('lastname'));

                throw Mage::exception(
                    'Mage_Customer', Mage::helper('customer')->__('This customer lastname already exists'),
                    Mage_Customer_Model_Customer::EXCEPTION_EMAIL_EXISTS
                );

    }

}
protected function HasLastName($lastname){


 $customer_check = Mage::getModel('customer/customer')
                ->getCollection()
                ->addAttributeToSelect('lastname')
                ->addAttributeToFilter('lastname',$lastname);

         if(count($customer_check)>0){
                return false;
         }  
         return true;     
}
}

Note: I have done basic of lastname field juts need to change field name to display_name