Remove duplicate arguments and order them via the cls's order_key key
    object/function.
    E.g.::

        >>> class Set(Operation):
        ...     order_key = lambda val: val
        ...     simplifications = [idem, ]
        >>> Set.create(1,2,3,1,3)
        Set(1, 2, 3)

def idem(cls, ops, kwargs):
    """Remove duplicate arguments and order them via the cls's order_key key
    object/function.
    E.g.::

        >>> class Set(Operation):
        ...     order_key = lambda val: val
        ...     simplifications = [idem, ]
        >>> Set.create(1,2,3,1,3)
        Set(1, 2, 3)
    """
    return sorted(set(ops), key=cls.order_key), kwargs