Ours did this and they are only 4 months - one weed and one pooed (so 2 nights running at the laundrette)! I have solved it by not letting them in the bedroom. I will let them in eventually but I have washed the duvet in disinfectant and when I let them back in I plan to put a blanket over the bed to make it seem different and also squirt with lemon or vinegar for a while. I think that they remember the places they have 'been' and will try to return to them. So I would advise shutting them out for a while, thorough cleaning with disinfectant and making it seem as different/unattractive as possible when you let them back in. I have heard that some cats just like to go on feather duvets so if all else fails you could consider replacing it with a different kind.
I know that shutting them out of the bedroom might be difficult - if you do this, buy earplugs for the first couple of nights, and whatever you do - don't open that door until morning so they get the message that wailing will not make you open it (you probably know that anyway)!
Let me know how you get on, I'm nervious about letting mine back in! By the way - other kittens that I had in the past used to like marking the duvet and all stopped in time, but we did have to buy a new duvet and carpet!