def cheap_factor(x):
    """
    Cheap prime factors without repetition
    >>> cheap_factor(91)
    [7, 13]
    >>> cheap_factor(12)
    [2, 3]
    """
    ret = list()
    i = 2
    while i <= x:
        if x % i == 0:
            ret.append(i)
            while x % i == 0:
                x //= i
        i += 1
    return ret