get your duvet washed with normal washing powder, if thats not possible and its wasnt too expensive bin it and buy a new one.
Some cats will pee on duvets or soft, comfy padding/blankets for no reason other than it feels nice. Once their smell is there even though we think its clean etc they will keep going back.
I'd recommend you find out if there are any medical reasons or is he stressed ??
In our case my cat just likes to urinate on bedding or when he's on the bed the urge just takes him..what ever the case is its extremely frustrating and annoying plus upsetting. We tried everything and the only thing that has worked was to shut cat in kitchen everynight and keep him out of the bedrooms altogether.
I'd highly recommend feliway products and with us the feliway spray was far more useful, after soiled areas were properly cleaned and dried i'd spray the areas with feliway.
I will say they do get better, in our case there hadnt really been incidents this year with cody spending more time unsupervised in the rooms (with kids its hard to keep him out of bedrooms) ... that was until this past week where there have been relapses for reasons i have no idea
hope you make some progress.