We sometimes close proposals summarily, due to being duplicates of others or generally being unacceptable. The problem is that those proposals disappear. Which means that someone else will come along and unknowingly propose them again.
We should let duplicates and "not a real proposal" proposals remain visible but closed. That way, people can see that we do not want them.
For example, consider these proposals:
All of these are (almost certainly) going to be closed as duplicates of SO (indeed, I'm rather surprised that they haven't been). But if these sites vanish after being closed, people will just try again, despite us clearly not wanting a site that detracts from SO.
We need to leave the closed sites around as object lessons.