On this question, I noticed that it was closed as a duplicate but the "Does this answer your question?" comment wasn't automatically removed like it's supposed to be, so I flagged it as "no longer needed." But my flag was declined, with no accompanying explanation. Was I wrong to flag the comment?
The question was not originally closed as a duplicate of the question that was in that comment, so the comment was not automatically deleted. I accidentally closed the question as a duplicate of an unrelated question, then reopened it when I noticed my mistake. This must've caused the original duplicate comment to stick even after the question was closed as a duplicate of the question in that comment.
Even when the question was closed, being able to see a different possible duplicate can still be helpful to anyone who visits the question, so I declined the "No longer needed" flag on the comment... because I thought it was referring to a different question. When looking at the question, I saw it was closed, I saw a comment with a possible duplicate, and I concluded, a little too quickly, that the comment was linking to a different question than the one that ultimately was voted for.
It's not entirely rare for "possible duplicate" comments to get flagged by people who believe the question is not actually a duplicate, so it seemed to me that this is what was happening here, and I didn't check to see whether the duplicate in the comment and the actual duplicate were different, because I didn't know they even could be. In cases like this with an exceptional circumstance, and where it's not immediately obvious what is no longer needed about a comment, a custom flag explaining your reasoning is also appropriate.
In short, that was my bad. I didn't see what was "No longer needed" about the comment because it was a situation I've never seen before. For these weird cases, custom flags can help us a lot.