Duplicate where original is deleted & wrongly marked in the first place

Question

This question is currently closed as a duplicate: On the index orderings for Christoffel symbols

That causes, as it does for all questions closed as a duplicate, a banner to display at the top that says "This question already exists" and then the name and link to the original question. The target of the duplicate, however, has been deleted, so the banner is not accurately reflecting the situation, except, perhaps, for high-rep users who can see deleted questions. (As far as I can tell, as below, it's actually been deleted twice.)

I submitted a moderator flag with this description:

This was closed as a duplicate, but the original question is now gone. The OP has attempted to edit and reopen by adding text to differentiate it from the (now gone) original question. Really this question should have been closed as non-mainstream in the first place, in my opinion, so I suggest that this either be deleted outright or the close reason changed so that it's eligible for Roomba.

At the time that I submitted my flag the original question had been (as far as I could tell) deleted by the OP. The OP then undeleted the original only to have it re-deleted by roomba later.

My flag was declined with this reason: "That Roomba deleted the original is irrelevant. It is still a duplicate." Honestly, I cannot make sense of that logic.

• Specific to this set of questions, it seems like both are off-topic (non-mainstream solicitation of private theories) and should both be closed in their own right. That will eventually lead the second question to roomba given the current votes. Preserving this specific question under its own circumstances seems to reward otherwise undesirable behavior. I think this one should be checked again even if it deviates from some broader general policy.
• In general, what's the guidance for this situation? There are limits on when and how targets of duplicates can be deleted to try to avoid this situation, but clearly it is still possible to occur. The general-level posts that I see are mostly about how and why to avoid it happening in the first place, not what to do about it when it happens.

Related:

• Cannot vote to close as a duplicate of a question that is already deleted, as discussed here: Closing Deleted Duplicates That makes sense because there's nothing for the duplicate to point to.
• Cannot delete one's own question if it is the target of a duplicate: How does deleting work? What can cause a post to be deleted, and what does that actually mean? What are the criteria for deletion?. This prevents such orphans from occurring by self-deletion.
• Related discussion on Math SE, with a query that can be modified to show how many orphans of this type exist in the system. https://math.meta.stackexchange.com/questions/32004/what-to-do-with-duplicates-of-deleted-questions
• A relatively old meta that pre-dates the 2015 decision to prevent targets of duplicates from being deleted (in most cases). It makes the point that seems to still stand that if the original is deleted, the duplicate should typically be deleted too. Does duplicate marking go away once original is deleted?

Answer 1

I wasn't the moderator who declined the flag, but I think I can tell you what happened here:

The situation we're dealing with here really is an edge case and not something that can generally happen - you've already linked a lot of info on dupe targets generally being prevented from being deleted. Additionally, normal users usually cannot choose a dupe target that has no upvotes or answers (and that's the reason the dupe target was deleted here by the roomba). That is, under ordinary circumstances, questions that qualify as roomba-deletion targets cannot be chosen as dupe targets.

The - now roomba-deleted - dupe target was posted by the same user, and the duplicate was closed as a duplicate by a moderator (hence overriding the usual rules about what is a valid dupe target). Essentially, this wasn't really a duplicate in the ordinary sense, but a user reposting their own question (slightly edited, but still it should probably have been an edit to the original post, not a new question).

Now, when the moderator declined your flag, they actually undeleted the dupe target! Unfortunately, this didn't save the dupe target from qualifying for roomba-deletion, so it was soon deleted again, probably before you saw that your flag was declined, hence you being confused about the decline reason - the intent here was "no, it's still a duplicate, look, the dupe target is back!". (Whether the flag should have been declined or marked as helpful instead since it provoked moderator action is debatable, but let's not sidetrack the discussion here)

Then the dupe target was again undeleted by a moderator, then deleted by roomba, etc. Even moderators don't know all the site's mechanisms by heart and I think the moderator simply expected that at some point their binding undelete vote would cause the roomba to stop. It did not. I think at this point we can say that as much as it would be desirable in a general situation to have the dupe target undeleted, we can't have that since we can't turn off the roomba for specific posts (and upvoting an off-topic question to preserve it also is not really what we want).

Especially since the duplicate currently also qualifies for roomba-deletion (no upvotes, no answers, closed) except for its age, the only two consistent options are:

1. Re-open the duplicate, then immediately close it with the close reason of the original dupe target.
2. Delete the duplicate, since the roomba will get it soon anyway.

Reopening and closing just for it to be roomba'd anyway is pretty pointless, so we should just delete the duplicate, too.