I was just looking at the flagged questions and noticed one was flagged as duplicate answer.
Other than the fact that I now think I was wrong to disagree with the flag, how does one flag an answer as a duplicate? I don't see it on the flag menu:
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The duplicate answers you find in the moderator tools are there not as the result of flagging, but because the system detected them automatically.
Moderators use this to hunt down spammers and other undesirable behaviour. If you feel the associated question is suitably different but merits the same answer, disagree with the flag to help lighten the moderator workload.