I posted a comment in How to convert 2's complement to its corresponding signed counterpart natural to the machine? to say it looked like it was a duplicate of Efficient unsigned-to-signed cast avoiding implementation-defined behavior. The question asker agreed with me and voted to close it as a duplicate. A month later I flagged it as a duplicate (I don't have vote-to-close privileges), and the flag was "deemed helpful". However two months later it still hasn't been closed, and according to Why do close votes expire? the close votes will have expired by now.
Should I flag it for "other (needs ♦ moderator attention)", or do something else, or just forget about it?
I also have a suggestion: Maybe when the person that asked the question votes to close it as a duplicate it should count as all 5 votes and the question should be closed immediately.
Edit: Thanks to Patrice for the information on the Community close votes. I guess my suggestion was redundant. However that was implemented after that question was voted to be closed as a duplicate by the OP, so I would still like to know what I should do.
You can't do anything and especially shouldn't flag for moderator attention, because that's not an exception that cannot be handled by the community even if the community is unwilling to handle it.
The SEDE query on this post tells you everything you need to know. It shows that 1 user voted to close, but 5 users skipped the review, presumably because they couldn't decide. The system decided not to show it to other users and your flag aged away.
Every action was done on 5th of february, so your flag was either on the same day or earlier. That was before the time that flags could be reused if they were not acted upon. So, your flag couldn't be given back.
Now, you can't do anything anymore. Please refrain from flagging for moderator attention (other), because that should only be done as an exception when the community cannot handle the issue. It doesn't give you a free pass to flag as such if the community is not willing to handle it.
The other feature that OPs can "accept" the duplicate was made available on 9th of february, so the OP couldn't yet close the question himself/herself.
How should I handle Exact Duplicate questions?
Cancel my vote-to-reopen a question flagged with a wrong duplicate?