For 3000+ rep users with the vote-to-close privilege, a duplicate flag is automatically turned into a duplicate vote-to-close. This makes perfect sense. However, as introducing the Marshal badge implicitly turned reaching a flag weight of 749 into an officially desirable goal ("Gold Badges are rare. You’ll have to actively work toward these. They’re something of an accomplishment!", cf. any badge site), this makes it even harder for users to reach that flag weight (≙636 successful flags).
It might not be so much of an issue at a huge site like SO, but on a smaller site like tex.sx it's virtually impossible to reach that flag weight this way. Tex.sx is pretty disciplined and I'd say most flags are dupe flags, the ones mentioned at Increasing Flag Weight when you have Voting Privileges? aren't too frequent.
My proposal, as a basis for discussion, is:
Increase a user's flag weight for a vote-to-close that contributed to the closing of a question as if it were a flag marked "helpful".