Why hibernate still checkUniqueness when update an entity

Hibernate ORM
1

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

DefaultSaveOrUpdateEventListener.performSaveOrUpdate
// 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
           this.entityIsDetached(event);
           return null;
       case PERSISTENT:
           return this.entityIsPersistent(event);
       default:
           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 
StatefulPersistenceContext
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.

2

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