This question came up in the VTC queue. It has been voted as having a duplicate already, but when I try to do the same, referring to the same duplicate, I'm not allowed to:
What am I doing wrong, that the other voter did right?
Update: It appears that a question can be flagged as a duplicate, but not closed as such, if the apparent duplicate does not have an upvoted answer. This looks like a bug.
That is strange indeed.
Using a low-rep sock, I tried to flag as duplicate (not vote to close) a different question as a duplicate of the same target and I was unable to. I got the same error message as you did.
I can see no indication that the target answers had been voted on (and then unvoted) in the timeframe, nor was either of them accepted then unaccepted. I can't explain how the flag to close as duplicate succeeded.
Anyway, I 'voted' to close as a duplicate now using my moderator account.
The close vote review queue also shows questions with dupe flags from users with less than 3,000 reputation points (who don't have the vote to close privilege).
If you go to the question directly, you can see that it doesn't have any active close votes. It seems that the restriction about dupe targets with no answers is not applied to flagging as a duplicate (which comes across like a bug to me, but who knows).
One possible case is to "abuse" the voting feature by:
This is sometimes done when the flagger/close-voter knows that the question is a dupe of another, but doesn't know if any of the answers on the dupe target are helpful or not.
Note that this is considered as an abuse to the system by working around the check on upvoted answers, and should be discouraged at all.
