The most common reason proposals are closed in the early stage is becuase they're a duplicate of an existing proposal/site. So why don't we have an option to flag proposals as a duplicate instead of flagging for 'moderator attention'?
Probably because it's not really necessary?
This isn't SO, where there are a bunch of 10k users who can act on a subset of the gazillions of flags they get. On big sites, yes, it's useful to have a separate 10k flag queue where you can review flags-as-duplicates while moderators handle different matters. On small sites like most of the current SE 2.0 sites and Area51 itself, there are few 10k users and the only difference between flagging as a dupe and flagging for moderator attention is (1) you type the words "duplicate of" instead of clicking a radio button and (2) there is no second difference.
Just flag for moderator attention and explain that it's a duplicate.