"Detached entity passed to persist" when entering the second time

I'm having an error when I try to insert a new user a second time.

Error:

12:51:54 PM com.sun.faces.lifecycle.InvokeApplicationPhase execute
WARNING: #{testeHibernate.save}: org.hibernate.PersistentObjectException: detached entity passed to persist: classhiber.Utilizador
javax.faces.FacesException: #{testeHibernate.save}: org.hibernate.PersistentObjectException: detached entity passed to persist: classhiber.Utilizador
    at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:118)
    at javax.faces.component.UICommand.broadcast(UICommand.java:315)
    at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:790)
    ... 24 more
javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:101)
    at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
    ... 26 more
Caused by: org.hibernate.PersistentObjectException: detached entity passed to persist: classhiber.Utilizador
    at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:141)
    at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:78)
    at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:843)
    at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:818)
    at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:822)
    at Impl.UtilizadorDAOimpl.addutilizador(UtilizadorDAOimpl.java:32)
    at TesteHibernate.save(TesteHibernate.java:48)
    ... 35 more

Class User:

@Entity
@Table(name = "utilizador")
public class Utilizador implements java.io.Serializable {

    private Integer idUt;
    private String user;
    private String pass;
    private String tip;

    public Utilizador() {
    }

    public Utilizador(Integer idUT, String user, String pass, String tip) {
        this.user = user;
        this.pass = pass;
        this.tip = tip;
    }

    @Id
    @GeneratedValue(strategy =GenerationType.AUTO)
    @Column(name = "id_ut", unique = true, nullable = false)
    public Integer getIdUt() {
        return this.idUt;
    }
    public void setIdUt(Integer idUt) {
        this.idUt = idUt;
    }

    @Column(name = "user", nullable = false, length = 100)
    public String getUser() {
        return this.user;
    }
    public void setUser(String user) {
        this.user = user;
    }

    @Column(name = "pass", nullable = false, length = 100)
    public String getPass() {
        return this.pass;
    }
    public void setPass(String pass) {
        this.pass = pass;
    }

    @Column(name = "tip", nullable = false, length = 100)
    public String getTip() {
        return this.tip;
    }
    public void setTip(String tip) {
        this.tip = tip;
    }
}

Adductor function:

    public void addutilizador(Utilizador ut) {
        System.out.println(this.sessionFactory);
        Session session = this.sessionFactory.openSession();
        Transaction tx = session.beginTransaction();
        session.persist(ut);
        tx.commit();
        session.close();        
}