Hibernate - a different object with the same identifier value was already associated with the session

Question

After changing the @id of a Entity from

@Id
private int getId(){
     return this.id;
}

to

@Id
private String getLogin(){
     return this.login;
}

I get the error:

a different object with the same identifier
value was already associated with the session

In the webapplication isn't changed anything. A read the entity and then change some fields in a form and than after submit I tried to save or update the Entity. With the int as @Id there was no problem but now with the String as @Id I get the above error by update or save the Entity:

 @Transactional(readOnly = false, propagation = Propagation.REQUIRES_NEW)
 public void saveOrUpdate(User u) {  
  getHibernateTemplate().saveOrUpdate(u);  
 }

What could be the problem?