title: Multiple representations of the same entity are being merged with @OneToMany
body: <p>I have a web application built with Spring Boot connected with a db PostgreSql, the project is about a education institute and manage Students and invoicing...</p>
  
  <p>I generate manually all the invoices. I add the first cuota(invoice) normally but when i want to generate the second, i've this issue:</p>
  
  <pre><code>java.lang.IllegalStateException: Multiple representations of the same entity [com.codeboros.app.entity.Cuota#1] are being merged. Detached: [com.codeboros.app.entity.Cuota@1788e1df]; Managed: [com.codeboros.app.entity.Cuota@2697e3fc]
  </code></pre>
  
  <p>I've this entities:</p>
  
  <pre><code>@Entity
  @Table(name="ALUMNOS")
  public class Alumno implements Serializable {           
      @OneToMany(mappedBy="alumno", cascade= {CascadeType.DETACH,CascadeType.PERSIST,CascadeType.DETACH,CascadeType.REMOVE,CascadeType.REFRESH,CascadeType.MERGE}, fetch=FetchType.LAZY)
      @NotFound(action = NotFoundAction.IGNORE)
      @JsonIgnore 
      private List&lt;Cuota&gt; cuotas;
  }
  
  @Entity
  @DiscriminatorValue(value="Cuota")
  public class Cuota extends Factura implements Serializable  {
      @ManyToOne(fetch=FetchType.LAZY)
      @JoinColumn(name="alumno_cod")
      @NotFound(action = NotFoundAction.IGNORE) 
      Alumno alumno;
  }
  </code></pre>
  
  <p>And the AlumnoController</p>
  
  <pre><code>@PostMapping("/alumnoGenCuota/{id}")    
  public String GenCuota(@PathVariable Long id, Cuota cuota) {
      Alumno alumno = alumnoService.get(id);
      cuota.setAlumno(alumno);
      cuota.setMonto(alumno.getCurso().getCuota()); 
      cuota.setDetalle(alumno.getCurso().getNombre()+":  $"+alumno.getCurso().getCuota()); //detalle
      alumno.AgregarCuota(cuota);             
  
      alumnoService.save(alumno);
  
      return "redirect:/alumnocuotas/"+id;
  }
  </code></pre>
  
  <p>I tried to remove CascadeType.MERGE but do not save the news Cuotas</p>