I know this is an old threat, but the topic is still very important. As a guest I find the thought of a duvet not being changed utterly discusting! In fact, I have just left an accomodation earlier than booked after finding out that they only change the top sheet, also now trying to get a refund.
Do you know what a guest has been doing on the duvet?
Not to mention that we Europeans do not like to sleep with the top sheet fixed at the side of the mattress, we want to be able to move at night. And the way I sleep, I touch the duvet often enough, does not matter if it has a top sheet.
Changing the duvet cover needs to be changed between every guest, no excuse please. I am in general not very fussy when it comes to accommodations, but when it comes to the cleanliness of the bed, I just cannot handle it not being changed