I’ve long found words with two consecutive u’s unsettling. It just seem wrong whenever to seem them together; I try to write the word vacuum as little as possible. I got to thinking about all this today, and after a little research learned that there are really barely any words that use the double u. Vacuum sure, continuum ok, muumuu I guess, and then the rest are very obscure. The ones you’ve never heard of are:
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
They all make sense and you could probably figure out their meaning on your own, but they’re still pretty rare.
Muumuu is Hawaiin and the German Weltanschauung (the world view of a particular individual or group) is used often enough to get a mention but I never could have summoned it up on my own.
There are people out there claiming some more but really they’re just Latin and I’m not buying.
The above info is from Oxford dictionaries