def ranges(iterable):
    delimiter = '-'
    iterable = iter(sorted(set(iterable)))
    stop = start = iterable.next()
    while True:
        try:
            cur = iterable.next()
            if cur - stop == 1:
                stop = cur
            else:
                yield yield_range(start, stop, delimiter)
                stop = start = cur
        except StopIteration:
            yield yield_range(start, stop, delimiter)
            raise StopIteration

def yield_range(start, stop, delimiter):
    if start == stop:
        return str(stop)
    else:
        return "%d%s%d" % (start, delimiter, stop)

if __name__ == '__main__':
    print ', '.join(ranges([1, 1, 2, 3, 5, 6, 11]))
    print ', '.join(ranges([1, 3, 4, 7, 7, 12, 13]))
    print ', '.join(ranges([1, 2, 3, 4, 7, 8]))
    print ', '.join(ranges([13, 17, 20, 27, 28, 29, 50]))