I always take two things into account when flagging a duplicate
1: For new (or relatively new) subscribers AU (to be helpful)
2: Those with rep (looking at their profile to see what questions they have asked recently to build a better picture what they are looking for)
For new subcribers when flagging a duplicate, I always make sure the target Q&A has been well asked and the answer will actually help them (without confusion). As well as adding a comment that states "Possible" or "Exact" duplicate" I will also explain the context. The assumption Im making here is that new subcribers dont realize their questions have been asked before. I know others take a more hard-lined view. I never point a new subscriber to an unanswered or badly answered duplicate. In this case I will coach them (in comments) to clarify their question better
For more experienced subscribers, I will comment "Possible duplicate" even if the target Q&A has not got a conclusive answer, and ask them to edit their question for additional clarity to ensure better success. In this case I would flag it to be merged. This sets an expectation with the OP that their question has been asked before with little or no success. I would expect them to go back and edit their question to be better than the original. (we live in hope :-) )
When in doubt I follow jrc's advice, head for chat room, and flag the abandoned question.