code: public <K, V> void assertDoesNotContainDuplicateValues(Description description, Map<K, V> actual) {
      assertNotNull(description, actual);
      Collection<?> duplicates = duplicatesFrom(actual.values());
      if (!duplicates.isEmpty()) {
        throw failures.failure(description, shouldNotHaveDuplicates(actual, duplicates));
      }
    }
docs: Asserts that the actual map does not contain the duplicate values.
  
  @param description contains information about the assertion.
  @param actual the given {@code Map}.
  @throws AssertionError if the actual map is {@code null}.
  @throws AssertionError if the actual map contains the given value.