Why hibernate still checkUniqueness when update an entity

when I update an object, I get a duplicate key exception, and debug code, I found this stack trace.

// RR become DETACHED state
protected Serializable performSaveOrUpdate(SaveOrUpdateEvent event) {
       EntityState entityState = this.getEntityState(event.getEntity(), event.getEntityName(), event.getEntry(), event.getSession());
       switch(entityState) {
       case DETACHED:
// go into this method
           return null;
       case PERSISTENT:
           return this.entityIsPersistent(event);
           return this.entityIsTransient(event);

// method2
protected void performUpdate(SaveOrUpdateEvent event, Object entity, EntityPersister persister) throws HibernateException {
       boolean traceEnabled = LOG.isTraceEnabled();
       if (traceEnabled && !persister.isMutable()) {
           LOG.trace("Immutable instance passed to performUpdate()");
       if (traceEnabled) {
           LOG.tracev("Updating {0}", MessageHelper.infoString(persister, event.getRequestedId(), event.getSession().getFactory()));
       EventSource source = event.getSession();
       EntityKey key = source.generateEntityKey(event.getRequestedId(), persister);
       // here throw exp
       source.getPersistenceContext().checkUniqueness(key, entity);
       if (this.invokeUpdateLifecycle(entity, persister, source)) {
           this.reassociate(event, event.getObject(), event.getRequestedId(), persister);
       } else {
           (new OnUpdateVisitor(source, event.getRequestedId(), entity)).process(entity, persister);
           source.getPersistenceContext().addEntity(entity, persister.isMutable() ? Status.MANAGED : Status.READ_ONLY, (Object[])null, key, persister.getVersion(entity), LockMode.NONE, true, persister, false);
           persister.afterReassociate(entity, source);
           if (traceEnabled) {
               LOG.tracev("Updating {0}", MessageHelper.infoString(persister, event.getRequestedId(), source.getFactory()));
           this.cascadeOnUpdate(event, persister, entity);

// method3 
public Object getEntity(EntityKey key) {
       return this.entitiesByKey.get(key);

the object which we want to update is in this entitiesByKey, so we get an exception

can someone tell why hibernate will have this check logic? Thanks in advance.

Post the code you are using and the stack trace of the error please.