'back' doesnt 'read' discussions

I'm not sure if this is to do with a change in the way the discussion list is created or presented or what, but if you click on a discussion, then click 'back' to the discussions list, the discussion remains unread. If you then refresh the page it turns read. Is this a browser setting or a bug or just the way things are now?
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<? Header('Cache-Control: no-cache'); Header('Pragma: no-cache'); ?>
Maybe you need to prevent browser caching on that page and any additional caching.