org.hibernate.PersistentObjectException: detached entity passed to persist: br.com.alura.jpa.modelo.Conta (?)

A tabela de conta não é criada e aparece essa exceção e não sei o que pode estar errado.

package br.com.alura.jpa.teste;

import br.com.alura.jpa.modelo.Conta;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;


public class CriaConta {

    public static void main(String[] args) {

        EntityManagerFactory emf = Persistence.createEntityManagerFactory("alura");
        EntityManager em = emf.createEntityManager();

        Conta c = new Conta();
        c.setTitular("Fernanda");
        c.setAgencia(3339);
        c.setNumero(1234);

        em.getTransaction().begin();

        em.persist(c);

        em.getTransaction().commit();

    }

}

o persistence:

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
        http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
    version="2.0">

    <persistence-unit name= "alura">
        <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
        <class>br.com.alura.jpa.modelo.Conta</class>

        <properties>
            <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
            <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost/alura_jpa" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password" value="" />

            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5InnoDBDialect" />
            <property name="hibernate.show_sql" value="true" />
            <property name="hibernate.format_sql" value="true" />

            <property name="hibernate.hbm2ddl.auto" value="update" />

        </properties>
    </persistence-unit>
</persistence>

e a class Conta:

package br.com.alura.jpa.modelo;
import javax.persistence.Basic;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Conta {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Basic(optional=false)
    @Column(name="id",unique=true,nullable=false)
    private Integer agencia;
    private    Integer numero;
    private String titular;
    private Long Id;
    private Double saldo;

    public Integer getAgencia() {
        return agencia;
    }
    public void setAgencia(Integer agencia) {
        this.agencia = agencia;
    }
    public Integer getNumero() {
        return numero;
    }
    public void setNumero(Integer numero) {
        this.numero = numero;
    }
    public String getTitular() {
        return titular;
    }
    public void setTitular(String titular) {
        this.titular = titular;
    }
    public Long getId() {
        return Id;
    }
    public void setId(Long Id) {
        this.Id = Id;
    }
    public Double getSaldo() {
        return saldo;
    }
    public void setSaldo(Double saldo) {
        this.saldo = saldo;
    }
}