Can I ask why would you want to? I would only iron a duvet cover for a visitor's room, far too busy with life to bother ironing such things otherwise. Fold them over and iron in sections if you really feel the need to.

I agree, I wouldn't want to iron a duvet or sheets. If I really think I need to get rid of the wrinkles, I toss it in the dryer with a damp cloth and a dryer sheet. Just remove it right before the cycle is over, shake it out, and put on the bed.