Hibernate: ConstraintViolationException with parallel inserts

I have a simple Hibernate entity:

@Entity
@Table(name = "keyword",
        uniqueConstraints = @UniqueConstraint(columnNames = { "keyword" }))
public class KeywordEntity implements Serializable {

    private Long id;
    private String keyword;

    public KeywordEntity() {
    }

    @Id
    @GeneratedValue
    @Column(unique = true, updatable=false, nullable = false)
    public Long getId() {
        return this.id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @Column(name="keyword")
    public String getKeyword() {
        return this.keyword;
    }

    public void setKeyword(String keyword) {
        this.keyword = keyword;
    }
}

DAO for it:

@Component
@Scope("prototype")
public class KeywordDao {

    protected SessionFactory sessionFactory;

    @Autowired
    public void setSessionFactory(SessionFactory sessionFactory) {
        this.sessionFactory = sessionFactory;
    }

    public KeywordEntity findByKeyword(String keyword) throws NotFoundException {
        Criteria criteria = sessionFactory.getCurrentSession()
                .createCriteria(KeywordEntity.class)
                .add(Restrictions.eq("keyword", keyword));
        KeywordEntity entity = (KeywordEntity) criteria.uniqueResult();
        if (entity == null) {
            throw new NotFoundException("Not found");
        }
        return entity;
    }

    public KeywordEntity createKeyword(String keyword) {
        KeywordEntity entity = new KeywordEntity(keyword);
        save(entity);
        return entity;
    }
}

and a service, which puts everything under @Transactional:

@Repository
@Scope("prototype")
public class KeywordService {

    @Autowired
    private KeywordDao dao;

    @Transactional(readOnly = true)
    public KeywordEntity getKeyword(String keyword) throws NotFoundException {
        return dao.findByKeyword(keyword);
    }

    @Transactional(readOnly = false)
    public KeywordEntity createKeyword(String keyword) {
        return dao.createKeyword(keyword);
    }

    @Transactional(readOnly = false)
    public KeywordEntity getOrCreateKeyword(String keyword) {
        try {
            return getKeyword(keyword);
        } catch (NotFoundException e) {
            return createKeyword(keyword);
        }
    }
}

In a single-threaded environment this code runs just fine. The problems, when I use it in multi-threaded environment. When there are many parallel threads, working the same keywords, some of them are calling the getOrCreateKeyword with the same keyword at the same time and following scenario occurs:

2 threads at the same time call keyword service with the same keyword, both first tries to fetch the existing keyword, both are not finding, and both try to create new one. The first one succeeds, the second - causes ConstraintViolationException to be thrown.

So I did try to improve the getOrCreateKeyword method a little:

@Transactional(readOnly = false)
public KeywordEntity getOrCreateKeyword(String keyword) {
    try {
        return getKeyword(keyword);
    } catch (NotFoundException e) {
        try {
            return createKeyword(keyword);
        } catch (ConstraintViolationException ce) {
            return getKeyword(keyword);
        }
    }
}

So theoretically it should solve the issues, but in practice, once ConstraintViolationException is thrown, calling the getKeyword(keyword) results in another Hibernate exception:

AssertionFailure - an assertion failure occured (this may indicate a bug in Hibernate, 
but is more likely due to unsafe use of the session)org.hibernate.AssertionFailure: 
null id in KeywordEntity entry (don't flush the Session after an exception occurs)

How to solve this problem?