public static FactoryBuilder newFactoryBuilder(Propagation.Factory delegate) {
    return new FactoryBuilder(delegate);
  }
@Test void dupesNotOk() {
    SingleBaggageField userIdConfig = SingleBaggageField.local(BaggageField.create("userId"));
    BaggagePropagation.FactoryBuilder builder = newFactoryBuilder(B3Propagation.FACTORY)
      .add(userIdConfig);
    assertThatThrownBy(() -> builder.add(userIdConfig))
      .isInstanceOf(IllegalArgumentException.class);
  }