DoubletonSetTest - Refactored

private void assertUnchanged() {
    Verify.assertSize(2, this.set);
    Verify.assertContainsAll(this.set, "1", "2");
    Verify.assertNotContains("3", this.set);
}

@Test
public void addDuplicate() {
    Throwable thrown = catchThrowable(() -> this.set.add("1"));
    assertThat(thrown).isInstanceOf(UnsupportedOperationException.class);
    this.assertUnchanged();
}