I was reviewing this question and flagged it as off-topic. A few minutes later I actually found an earlier duplicate, much better formulated and with better answers. I tried opening the "flag" menu again, but the "duplicate" flag is greyed out and I get the "you have already raised this type of flag" message.
Why can't I change it within a few minutes of noticing my mistake?
Good question, I suppose you can't 'change' your flag once it's submitted. In a case like this, I'd just hit the 'moderator intervention' reason and explain what's up. We can then fix things for you.
I don't think mods can change the flag anyway (correct me if I'm wrong @mods). If you find a duplicate after you've already flagged a close reason, the best option is to just leave a comment like:
"Possible duplicate of [question title](question url)"
When it shows up in the review queue it should show that as a possible dupe (I think), at the very least it is visible to anyone voting to close (either through review or directly from the question).
