Returns whether any of the Ts are duplicate w.r.t. hana::equal.
In particular, this does not check whether all of the Ts are unique as types, but rather whether they are unique when compared as hana::equal(std::declval<T>(), std::declval<U>()). This assumes the comparison to return an IntegralConstant that can be explicitly converted to bool.
false most of the time (otherwise we will assert). Hence, this implementation is biased towards the fact that we will have to compare every pair of elements in most cases, and it does not try to be lazy.