Now that we finally have the wonderful Case Event Merge plugin, choosing a duplicate becomes more important, especially the right one.
Best would be for FogBugz to choose a duplicate automatically. Including checking the Show in other case box and marking the duplicate unread.
Else, maybe FogBugz could suggest duplicate cases once I Resolve as Duplicate. Based on whatever email header is useful and/or the correspondent and/or the text content. That would be way better than having to manually search for that one specific case number myself...