I'm in the US and we use a flat sheet between us and the duvet cover. We wash the duvet cover but the flat sheet gets washed more often and is less expensive to replace, and is replaced more often (nothing gets replaced that often, but relatively speaking).

I don't think there's really any right way of doing things though. Sometimes I wonder if we should just have a linen blanket or something on top in the summer but we never seem to get too hot somehow, and I've never managed to find a top blanket that is the right size and that we like.

(I'm also not really sure I want to store our duvet just for the summer either but that's solvable.)