Hi,
For that, you will probably have to edit the class "user" in "administrator/components/com_hikashop/classes/user.php", function "register()" and edit the following if condition:
		if($simplified==2){
			$this->database->setQuery('SELECT * FROM '.hikashop_table('user').' WHERE user_email = '.$this->database->Quote($userData->user_email));
			$userInDB = $this->database->loadObject();
			if(@$userInDB->user_cms_id){
				JError::raiseWarning('', JText::_('EMAIL_ADDRESS_ALREADY_USED'));
				return false;
			}
			$this->user_id = 0;
			if(isset($userInDB->user_id)){
				$this->user_id = $userInDB->user_id;
			}
			if(!$this->user_id){
				$this->user_id = $this->save($userData);
			}
			//unpublish previous addresses so that there is no information leakage
			$query = 'UPDATE '.hikashop_table('address').' AS a SET a.address_published=0 WHERE a.address_user_id='.(int)$this->user_id.' and a.address_published=1';
			$this->database->setQuery($query);
			$this->database->query();
		}
Be careful it's a core edition, you change will be lost at each update. And it require PHP knowledge.