Hi Richard,
In order to understand what's happening here (since the "Request Not Processed" error is generic and doesn't really tell us what's going on), add &mode=xml at the end of the string in your address bar while on the page before duplicating your event, and hit enter. The page should refresh and show a white box at the bottom of the screen with some XML data in it. Now try duplicating the event- the white box should show the full, exact error message behind "Request Not Processed".
Also, this used to happen when you duplicated an event which was in some way broken - eg, the Meeting room or Content the source event is linked to was deleted.
Thanks!