Answer
Common: vacuum, vacuumed, vacuuming, vacuums.
Less common: continuum, continuums, muumuu, muumuus.
Much less common: carduus, carduuses, continuua, duumvir, duumviral, duumvirate, duumvirates, duumviri, duumvirs, individuum, individuums, lituus, lituuses, menstruum, menstruums, mutuum, mutuums, paramenstruum, paramenstruums, residuum, residuums, squush, squushed, squushes, squushier, squushiest, squushing, squushy, triduum, triduums, ultravacuum, ultravacuums, weltanschauung, weltanschauungen, weltanschauungs.
