Regularly air your duvet and pillows outdoors – especially on hot, dry, sunny, breezy days. This should be enough to keep your bedding fresh, especially if you frequently change your pillowcases and covers.
If required, dry clean gently in low temperatures. Make sure that your dry cleaning service follows instructions for woollen products.
Warning: do not subject to excess heat at any time, as this may ruin your duvet and void your warranty.
Note that the cold machine setting that works for your wool sweaters is actually not recommended, because the agitation alone may shift or shrink the fibres inside your duvet. Spot-cleaning with cold water and flat-drying is fine.