What is an equivalent of I just crossed the 3K threshold! Any tips for responsibly wielding this power? for Area 51?
Whenever I close a proposal (particularly duplicates), sometimes I take a wait-and-see approach. There might be a good reason to have a duplicate proposal. Perhaps there is a unique aspect to the proposal that the author wishes to pursue. That doesn't mean I have to agree with him. Those cases are usually decided by the (lack of) support by the community.
Maybe the author is better suited to launching a successful campaign: If the author is some world-class maven in [subject] who wrote a better proposal, asks better questions, rallies more support, blogs about it enthusiastically, and it takes off. Proposals that look the same aren't necessarily equal. So I give it a bit of a chance to see where it goes.
But I stay on top of it and check back often. Sometimes a duplicate proposal was placed inadvertently (the search didn't bring up the duplicate). In those cases, the author is usually more than happy to support the other proposal. If nothing special happens with the proposal I'm watching, there's no use in splitting up the subject, so I close it.
I usually leave a comment indicating that the author may follow up with a reason they placed the apparently-errant proposal.
It should be pretty obvious what kind of proposals need to be closed, like obvious duplicates or completely ridiculous joke proposals. The only grey area here is that what might seem to you like a spurious proposal might not actually be so, so I'd at least leave a comment on what might look like a dubious case to check and make 100% sure before throwing your weight around.