The OnBeforeDocDuplicate and OnDocDuplicate events are missing from manager/processors/duplicate_content.processor.php
Entries must be added to the system_eventnames table:
INSERT INTO `modx_system_eventnames` (`id`, `name`, `service`, `groupname`)
VALUES
(1054, 'OnBeforeDocDuplicate', 5, ''),
(1055, 'OnDocDuplicate', 5, '')
(The id numbers above are arbitrary)
See also MODX-189, MODX-308, MODX-312