Entity Framework 6: Error saving - Instances of Ientitychangetracker

Question

Domain

public class Agenda
{
    public int Id { get; set; }

    public DateTime Data { get; set; }

    public string HoraInicial { get; set; }

    public string HoraFinal { get; set; }

    public string Local { get; set; }

    public virtual ICollection<Treino> Treinos { get; set; }
}

public class Treino
{
    public int Id { get; set; }

    public string Objetivo { get; set; }

    public string Descricao { get; set; }

    public virtual ICollection<Agenda> Agendas { get; set; }
}

Configurations of Entities

public class TreinoMap : EntityTypeConfiguration<Treino>
{
    public TreinoMap()
    {
        HasMany(x => x.Agendas)
            .WithMany(x => x.Treinos)
            .Map(x => 
            {
                x.MapLeftKey("Treino_Id");
                x.MapRightKey("Agenda_Id");
                x.ToTable("AgendaTreino");
            });
    }
}

public class AgendaMap : EntityTypeConfiguration<Agenda>
{
    public AgendaMap()
    {
        HasMany(x => x.Treinos)
            .WithMany(x => x.Agendas)
            .Map(x =>
            {
                x.MapLeftKey("Agenda_Id");
                x.MapRightKey("Treino_Id");
                x.ToTable("AgendaTreino");
            });
    }
}

Method I use to save

public void Inserir(T obj)
{
    banco.Set<T>().Add(obj);
    banco.SaveChanges();
}

The Agenda object is completed as follows: I normally fill all properties via parameter, with the exception of Property Trainings, it comes null. But in Scheduleviewmodel, I get the training descriptions in a string and convert to Array. Then I pass this array to the method and receive all the Training objects. And, from there, I link these workouts to my Schedule, and, finally, I send save. Below is the treatment of the object.

public ActionResult NovaAgenda(AgendaViewModel agenda)
{
    var descricoesDeTreinosEscolhidos = agenda.TreinosEscolhidos.Split(',');
    var treinosDomain = _treinoApp.ObterTreinosPorDescricao(descricaoDeTreinosEscolhidos) as IEnumerable<Treino>;

    var agendaDomain = MapearAgendaViewModelParaAgenda(agenda);
    foreach(var treino in treinosDomain)
    {
        agendaDomain.Treinos.Add(treino);
    }

    _agendaApp.Inserir(agendaDomain); // Aqui que lança a exceção.

    return View();
}

The following exception is thrown (I could not format with the vertical scroll bar):

An Entity Object cannot be referenced by Multiple instances of Ientitychangetracker. in System.Data.Entity.Core.Objects.ObjectContext.Verifycontextforaddorattach(Ientitywrapper wrappedEntity) em System.Data.Entity.Core.Objects.ObjectContext.AddSingleObject(EntitySet entitySet, IEntityWrapper wrappedEntity, String argumentName) em System.Data.Entity.Core.Objects.DataClasses.RelatedEnd.AddEntityToObjectStateManager(IEntityWrapper wrappedEntity, Boolean doAttach) em System.Data.Entity.Core.Objects.DataClasses.RelatedEnd.AddGraphToObjectStateManager(IEntityWrapper wrappedEntity, Boolean relationshipAlreadyExists, Boolean addRelationshipAsUnchanged, Boolean doAttach) em System.Data.Entity.Core.Objects.DataClasses.RelatedEnd.IncludeEntity(IEntityWrapper wrappedEntity, Boolean addRelationshipAsUnchanged, Boolean doAttach) in System.Data.Entity.Core.Objects.DataClasses.Entitycollection1.Include(Boolean addRelationshipAsUnchanged, Boolean doAttach) em System.Data.Entity.Core.Objects.DataClasses.RelationshipManager.AddRelatedEntitiesToObjectStateManager(Boolean doAttach) em System.Data.Entity.Core.Objects.ObjectContext.AddObject(String entitySetName, Object entity) em System.Data.Entity.Internal.Linq.InternalSet1.<>c__DisplayClassd.b__c() in System.Data.Entity.Internal.Linq.Internalset1.ActOnSet(Action action, EntityState newState, Object entity, String methodName) em System.Data.Entity.Internal.Linq.InternalSet1.Add(Object Entity) in System.Data.Entity.Dbset1.Add(TEntity entity) em ProjetoTeste.Infra.Data.Repositories.RepositoryBase1.Insert(T obj) in D: Informática Raphael Projects VS2015 Projectthis Code Projectthis Projectste.Data Repositories Repositorybase.Cs:line 54 in Projetoteste.domain.Services.Servicebase1.Inserir(T obj) na D:\Informática\Raphael\Projetos VS2015\ProjetoTeste\Codigo\ProjetoTeste\ProjetoTeste.Domain\Services\ServiceBase.cs:linha 24 em ProjetoTeste.Application.Services.AppServiceBase1.Insert(T obj) in D: Raphael Informatics Projects VS2015 Projectthis Code Projectthis Projectste.Application Services Appservicebase.Cs:line 28 em ProjetoTeste.Presentation.Controllers.AgendaController.NovaAgenda(AgendaViewModel agenda) na D:\Informática\Raphael\Projetos VS2015\ProjetoTeste\Codigo\ProjetoTeste\ProjetoTeste.Presentation\Controllers\AgendaController.cs:linha 69