This very much depends on what kind of "duvet" you are dealing with. If it has synthetic filling then by all means wash it, but if it has some form of natural feather then do not contemplate washing it. If it has down (swan, goose, common duck or eider in ascending value) then you should really have your duvet cleaned and sorted by a specialist. I know of no such specialist in Britain although they may well exist. They remove the filling, separate and discard fragments and add similar fill if too much loss has occurred to maintain the quality of function. The best centres for treating natural fill duvets are probably to be found in the Nordic countries where the tradition of using these without any other cover (sheet, blanket etc.) over you originated. If you have never had the real thing, then you should try it - stick to a single sized one whether you sleep alone or not (it will not at all inhibit anyone seriously contemplating conjugal acts - if it should you know he/she lacks the passion or has some sort of hangup). The proper thing (at least goose down if you can't afford eider - no feather mixtures ever, not if you are serious) is a truly life changing luxury - you will find them surprisingly realistically priced in downtown Reykjavik (Dun & Fidur). Treat them well and you can go ten years without any cleaning/servicing.