code: @Test public void should_fail_if_actual_contains_given_values_more_than_once(){
    AssertionInfo info=someInfo();
    actual.addAll(newArrayList("Luke","Luke",null,null));
    Object[] expected={"Luke","Luke","Yoda","Han",null};
    try {
      iterables.assertContainsOnlyOnce(someInfo(),actual,expected);
    }
   catch (  AssertionError e) {
      verify(failures).failure(info,shouldContainsOnlyOnce(actual,expected,newLinkedHashSet("Han"),newLinkedHashSet("Luke",null)));
      return;
    }
    failBecauseExpectedAssertionErrorWasNotThrown();
  }
  