EF Multiple added entities may have the same primary key error

Question

I'm going to create a new cascade [node] --1:many-- [currency] --1:many-- [unit] defined using EF 6.x Code First:

public class CashFlowSpendableMinimumNode {
  ...
  [ForeignKey("CashFlowSpendableMinimumNodeId")]
  public ICollection<CashFlowSpendableMinimumCurrency> CashFlowSpendableMinimumCurrencies { get; private set; }
  ...
}
public class CashFlowSpendableMinimumCurrency {
  ...
  public int CashFlowSpendableMinimumNodeId { get; set; }
  [ForeignKey("CashFlowSpendableMinimumNodeId")]
  public CashFlowSpendableMinimumNode CashFlowSpendableMinimumNode { get; set; 
  ...
  [ForeignKey("CashFlowSpendableMinimumCurrencyId")]
  public ICollection<CashFlowSpendableMinimumUnit> CashFlowSpendableMinimumUnits { get; set; }
  ...
}

public class CashFlowSpendableMinimumUnit {
  ...
  public int CashFlowSpendableMinimumCurrencyId { get; set; }
  [ForeignKey("CashFlowSpendableMinimumCurrencyId")]
  public CashFlowSpendableMinimumCurrency CashFlowSpendableMinimumCurrency { get; set; }
  ...
}

The first part i.e. relation between node--currency work as expected: I create a new currency instance and add it to DbSet<CashFlowSpendableMinimumCurrency> -> it appears in the navigation property (collection) nodeMinimum.CashFlowSpendableMinimumCurrencies and its Id is resolved properly when relation is saved to DB.

The second relation currency--unit does not work even if I tried to define it the same way as the first relation between node--currency: when I create a new unit instance and add it to DbSet<CashFlowSpendableMinimumUnit> it does not appear in the navigation property currMinimum.CashFlowSpendableMinimumUnits and if I continue execution, an attempt to make cascade persistent by calling SaveChanges() later results in the error in the %subject%.

UPDATE 1

I omit exact wording of error message, here it is:

Unable to determine the principal end of the 'xxx.yyy.Database.CashFlowSpendableMinimumNode_CashFlowSpendableMinimumCurrencies' relationship. Multiple added entities may have the same primary key.

UPDATE 2

The only solution to make it running I've discovered is to flush the change tracker prematurely:

foreach (var node in nodes)
{
   nodeMinimum = new ...
   DbSet<CashFlowSpendableMinimumNode>.Add(nodeMinimum);
   foreach (var curr in currencies)
   {
      currMinimum = new ...
      DbSet<CashFlowSpendableMinimumCurrency>.Add(currMinimum);

      DbContext.SaveChanges();  //The premature flush to fix a problem

      foreach (var unit in units)
      {
         unitMinimum = new ...
         DbSet<CashFlowSpendableMinimumUnit>.Add(unitMinimum);
      }
   }
}
DbContext.SaveChanges();
...

UPDATE 3

Concerning discused constructors and instance creation:

public CashFlowSpendableMinimumCurrency()
{
  CashFlowSpendableMinimumUnits = new List<CashFlowSpendableMinimumUnit>();
}

[...]
CashFlowSpendableMinimumCurrency result = new CashFlowSpendableMinimumCurrency()
{
  CurrencyCode = ACurrencyCode,  // the attached entity
  Curr = ACurrencyCode.Curr,     // string
  Minimum = minimum,  //decimal
  Median = median,  //decimal
  CashFlowSpendableMinimumNode = ANodeMinimum,  // parent in 1:n relation, EntityState.Added
  ExchangeRate = 1, //decimal
  DeclaredExchangeRate = false,
  BanknotesContribution = 0, //double
  BanknotesSpendableMinimum = 0, //decimal
  CoinsContribution = 0, //double
  CoinsSpendableMinimum = 0, //decimal
};
ADbContext.CashFlowSpendableMinimumCurrencies.Add(result);

Please, does anybody see, what is wrong?

Thanks, pf