This is a proposal for more obvious presentation of marking, and unmarking, duplicate bug reports.
If a bug report has duplicates of its own, it should have a "This report has n duplicates" section that expands to list those duplicates.
Otherwise, if it is not already a duplicate, it should have a "Mark as a duplicate" section that expands to show the form for marking it as a duplicate. The field for entering the bug number should automatically be focused.
Otherwise, it is already a duplicate, so it should have a "This is a duplicate of {number}: {summary}" section that expands to show a form for changing the duplicate marking. The field for entering a new bug number should automatically be focused.
In either case, if the form is submitted with an error, the expandable section should still be expanded when the page reloads.
Without JavaScript, "This report has n duplicates", "Mark as a duplicate", and "This is a duplicate" should not be links. Instead, the duplicate section should be displayed inline, and "This report has n duplicates" and "Mark as a duplicate" (but not "This is a duplicate") should each be followed by a colon.