Puzzle ZIWQ 

Workings

Hint
Some are much more common than others.

Answer
Continuum, continuums, residuum, residuums, vacuum, vacuums, vacuumed, vacuuming, muumuu, muumuus.

There are a few more less common words such as menstruum, menstruums, triduum, triduums, weltanschauung, weltanschauungen.