@Test public void noChildrenWithSameName() {

  MortarScope rootScope = scopeBuilder.build("Root");
  rootScope.buildChild().build("childOne");
  try {
    rootScope.buildChild().build("childOne");
    fail();
  } catch (IllegalArgumentException e) {
    assertThat(e).hasMessageContaining("already has a child named");
  }
}