We've one-upped them all, because over here duvets are unheard of, let alone duvet covers :D
But seriously, I (unfortunately?) live in a hot and humid part of the world and I suppose because of that duvets and its covers are unnecessary here. We've just got bedsheets, blankets, pillow-covers.
This reminds me. I went to university in America. From a Walmart nearby I bought one of those packaged set of things you put on a bed — "comforter", "bedspread", bedsheet, something with an elastic around it, pillow covers etc. I know it was these things because the cover had them written on it.
I unpacked them, and I have no idea which one is a comforter, which one is a bedspread etc. Nor did I know the order of how I put these onto the bed, what one goes on top of what other thing etc. And I didn't know in between which layers I should be sleeping in.
My not-very-friendly roommate and his girlfriend probably thought I was a moron for not knowing this.