Spot clean stains using damp cloth + mild detergent.
Dry clean as needed (1x/year if using a cover).
Duvet Cover recommended—extends life and keeps it clean longer.
To naturally freshen & fluff
Air out duvet (hang out in the sun for a few hours) and shake it out.
In a nutshell
A duvet/comforter is like a pillow. You don't need to clean it often if using a cover, and you can easily maintain it with some time outside and shaking it out.
More tips: How to treat stains | How to brighten white linens naturally