public void testPuttingTheSameKeyTwiceThrowsOnBuild() {
    Builder<String, Integer> builder =
        new Builder<String, Integer>()
            .put("one", 1)
            .put("one", 1); // throwing on this line would be even better

    try {
      builder.build();
      fail();
    } catch (IllegalArgumentException expected) {
      assertThat(expected.getMessage()).contains("one");
    }
  }