Don't wash them if you can at all avoid it!
It's really rather simple but quite unusual sometimes to British people! However if you go to China, or indeed see pictures of residential areas in China, you will see this as they do it with all their duvets!
On a nice sunny day put the duvet in the sinlight with maximum exposure to the sun. You will find this revitalises and refreshes the duvet. Chinese people are adament you can tell the difference after a duvet has been in the sunshine all day long.
Give maximum exposure to all parts of the duvet. After a couple of hours, turn it round or flip it over, or whatever you need to do.
Finally, go to bed underneath your hypoallergeric, meltingly soft and gently light duvet.