Transaction with Entity Framework. Saves 2 times after error

Question

I am using transaction to make changes to the bank. My problem is when there is an error in saving time.

If there is error at the time of inserting the drive it treats the correct way not saving anything in the bank. However, when it comes back to the screen, I fix the problem (which was the size of the field at the base) and insert it again, only it records the database records twice.

    public override void Add(NotaFiscalItem obj)
    {
        using (var transaction = Db.Database.BeginTransaction())
        {
            try
            {
                ItemMovimentacao mov = new ItemMovimentacao();
                mov.Numero_Item = obj.Numero_Item;
                mov.Origem_ItemMovimentacao = "Inclusão de Item de NF";

                Db.NotaFiscalItens.Add(obj);
                Db.ItemMovimentacoes.Add(mov);

                Db.SaveChanges();

                transaction.Commit();
            }
            catch (Exception)
            {
                transaction.Rollback();
                throw;
            }
        }
    }

That is, it generates the expected error for the overflow problem in the bank, but, when it goes back to the screen I have run again (already with the problem solved), it records again what he could not record the first time.