We have 2 we use on the top of our bunk bed that i have washed MANY times with great luck:

A down comforter from Garnet Hill. LOVE these - we bought the dark colored ones so no duvet necessary and it IS very soft cotton. worth every penny... and have washed quite a few times and it comes out great.

and i have to say i have washed our IKEA comforters quite a few times and they still look great - but don't believe they are cotton.