@Test public void dupesNotOk() {

  SingleBaggageField userIdConfig = SingleBaggageField.local(BaggageField.create("userId"));
  BaggagePropagation.FactoryBuilder builder = newFactoryBuilder(B3Propagation.FACTORY)
    .add(userIdConfig);
  assertThatThrownBy(() -> builder.add(userIdConfig))
    .isInstanceOf(IllegalArgumentException.class);
}