Contrary to what is often stated, the down duvets and other high quality down /feather products, are not directly responsible for house dust mite allergy. Goose down and feathers, do not cause allergic reactions in the airways (asthma, hay fever, etc …)
The filling of our feathers and down  has been sterilized at a temperature of 120 ° (Danish regulations BS 1425), and in addition, tested and checked by local  Danish veterinary authorities all through the production process according  European legislation (Denmark has more  severe laws in this matter  than  other  European countries) 
Furthermore the woven density of the fabric which has to guarantee for years that the filling will not come out,  also guarantee that the dust mies cannot penetrate inside the duvet. 
The thermal properties of the goose down: rapid evolution of warmth during sleep and rapid dispertion of excess  moisture into the external environment, makes the bed “cave”, comfortable for humans, and inhibits the proliferation of dust mites .
Thus are no practical and scientific reasons which prevent persons with house dust mite allergies to  sleep under  high quality down  duvets. Except the very few people with specific down allergy.
The use of duvets in conjunction with duvet covers, which are regulary washed, improves  hygiene. Blankets used with a top sheet leave the blanket exposed to dust accumulation.