When a task is marked as a duplicate and merged, it should be visually clear that such task is over, and that the discussion is now happening somewhere else. Currently duplicates look very similar to open tasks. Especially in the eyes of untrained users, who see in the first place an open request or even an ongoing conversation, and (in our short experience) some do keep posting in good faith, missing the .
* "Closed, Duplicate" is indeed there on top, but maybe it could benefit from some UX emphasis. Maybe.
* "Qgil merged this task into..." appears indeed in the log as an action, with a lightgrey-white icon, but that is easy to miss, especially if other actions follow (for instance, a mention in the destination task is likely to come).
While changing the design of "Closed, Duplicate" has other implications, putting more stress in the "merged this task" line shouldn't be problematic. A boldly colored icon would help already.