I've marked this question a duplicate. Then I explained it in a comment. After the submission of that comment another comment appeared above it ("possible duplicate of...") with my username. So deleted this, because that was less informative.
Then I noticed an empty pair of round brackets "()" next to "flag". I've clicked on it, and noticed, that the status was "you have already flagged this" for both "duplicate" and "another reason".
How have I given two flags at a time? Why is that? Is this a bug (I'll change the tag if it is)?