My continental rellies use a tucked-in sheet under the duvet, and the duvet cover has a sort of tongue that tucks in under the mattress. To keep it in its place. All of which, to my mind, negates some of the advantages of the duvet, not least its lightness and kick-off-ability.
And then when it comes to laundry the top and bottom sheet get done weekly but the duvet cover only so often. Is this a reasonable way of living, I ask you.