There are two types of duvet and I shall look at each

Synthetic Duvets

These contain manmade material and are not a problem to clean. If your washing machine is big enough you can wash them in there. Do not go through the spin cycles as the weight of the duvet could send your machine walking. As a general rule I take the familu duvets to the launderette / Laundromat. There the machines are big enough to wash and dry your duvets in one go.

Feather / Down Duvets.

If you spill something on your feather duvet mop up the stain as soon as possible. Feathers or down should be pushed away from the stained area as much as possible. You can then wash the stain with a mild detergent, avoid making the stained area too wet. Larger stains need to be dry-cleaned. As a general rule keeping a washable cover on the duvet could avoid further stains.