Are you referring to the (inner) duvet, or the (outer) duvet cover?
Add a complimentary colour along the sides (length) and then make a pillow to match to pull it all together.
Add a contrast strip to either side of the duvet or Cut it down the middle and add a panel there. Easy fix.
You will probably need to add to at least three of the sides since queen size is both longer and wider than a double.