Words which contain 'u' twice with no intervening letter are rare, indeed there are only eight proper English words (most of which come from Latin) and another two borrowed from other languages.
vacuum = a space
continuum = a continous sequence
residuum = a chemical residue
menstruum = the matter discharged during menstruation
triduum= a three-day period of religious observance in the Catholic Church
duumvir = each of a pair of magistrates holding joint office in ancient Rome
duumvirate = a coalition of two people having joint authority
Weltanschauung = the world view of a particular individual or group (from German)
muumuu = a loose dress traditionally worn in Hawaii (from Hawaiian) which qualifies twice!