Now, when reviewing reopen votes on questions closed as duplicates, I've got no link anywhere to the question that the closed question is a duplicate of (i.e. the "duplicated" question):
Needless to say, without access to the duplicated question it is impossible to judge whether the question being reviewed is truly a duplicate or not, and hence impossible to judge whether a reopen is required or not.
Interestingly, that particular question has now been reopened. I wonder if the absence of a link to the duplicated question resulted in a sufficient number of "I don't see a duplicate - I better reopen it" type reviews.
It looks like I'm not the only one to be bitten by this issue: LittleBobbyTables was busy writing this comment while I was writing this question.