List<ClassName> filteredObjects = objects.stream()
    .collect(Collectors.groupingBy(ClassName::getSecNumber))
    .values().stream()
    .flatMap(os -> os.stream().anyMatch(o -> o.getType() == Type.NEW) ?
        os.stream().filter(o -> o.getType() != Type.LEGACY) :
        os.stream()
    ).collect(Collectors.toList());

Collection<T> result = list.stream()
    .collect(toMap(T::getSecNumber,
            Function.identity(),
             (l, r) -> l.getType() == Type.LEGACY ? r : l))
    .values();

List<T> result =
            list.stream()
                .collect(groupingBy(T::getSecNumber))
                .values()
                .stream()
                .flatMap(l -> l.stream().anyMatch(e -> e.getType() == Type.NEW) ?
                    l.stream().filter(e -> e.getType() != Type.LEGACY) :
                    l.stream())
                .collect(toList());