How to avoid "a different object with the same id.."?

I am using:

• Web App (a filter opens session. DAO uses getCurrentSession())
• Hibernate
• Spring (AOP configuration over Service)
• xml configuration for all
• DTO between Mbean and Service

Well, I have two methods (business service):

• service.findUser(..DTO..)
• service.updateUser(..DTO..)

update throws org.hibernate.NonUniqueObjectException exception. How can I avoid that? I need to use update, not merge.

Thanks in advance