It is listed but in a very difficult to discover way: https://phabricator.wikimedia.org/T857#14531

Case in point: https://phabricator.wikimedia.org/T54136

In T883#780224, @Nemo_bis wrote:

Case in point: https://phabricator.wikimedia.org/T54136

Which case? Having to scroll the ticket and look out for "has been marked as a duplicate of this bug" Bugzilla comments? That they are not somewhere at one place listed? That they are not linked?

Could someone please provide a clear user story with a resulting problem? I don't see yet which of all these aspects is the actual issue described here. Plus first comment was about tasks created in Phab merged into tasks created in Phab. Link provided above is about imported Bugzilla tasks hence also a different topic and behavior...

Done.

Not really... "I want to have a list because I want to have a list" is not a helpful usecase to me if I have no idea why you want that list...

I think the point is that sometimes a task is marked as duplicate when it describes a special case of another task. Knowing these special cases might be helpful, so i think having a list would be a good thing.

One problem of the current workflow is that tasks merged are logged in the history of the task, but no action is logged if the task is unmerged.

I think it is worth giving this request a shot and see what upstream thinks about it.

"Show merged tasks" in upstream is merged into "Adding related tasks" which is T76101 in our Phabricator instance.
Argumentation: If such information was provided, people would want to know all edge relationships (dups, blocked, seealso, etc) in general anyway.

This is probably trivially easy, similarly to {D239}

Now that a "Related Objects" box exists, the merged task and the one receiving it reference to each other under "Mentioned in", but without any identification showing the "merged" relationship (see i.e. this and this tasks).

With the UI resolved, now this problem is perhaps more trivial to solve?

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https://secure.phabricator.com/T12234 got resolved as fixed so we will also see this fixed in Wikimedia Phabricator next time we pull from upstream.