Collectors.reducing to List

You can use Collectors.toMap():

List<User> l = new ArrayList<> (list.stream()
    .collect(Collectors.toMap(u -> u.name,
                              u -> new User (u.name,u.languages),
                              (u1, u2) -> new User(u1.name, u1.languages + ", " + u2.languages)))
    .values());