ConcurrentModificationException (invalid foreach iterator)

[FreekMank]

Adding the same exchange rate twice throws a java.util.ConcurrentModificationException
(because the foreach iterator is invalidated because an element is removed)

Calculator calc = Calculator.create();
calc.addExchangeRate(ExchangeRate.create("CZK", "USD", 0, 12));
calc.addExchangeRate(ExchangeRate.create("CZK", "USD", 17, 1));