phranque - 8:36 am on Jul 27, 2011 (gmt 0)

They could be both, with different Discovery Dates.

that's possible, but i would guess that one would cancel out the other upon discovery or shortly thereafter.

If the pages don't exist, and clicking on the links doesn't lead to anything, where are the duplicate titles coming from? It seems a little unfair to cry Duplicate Title when the title in question is "404--Page Not Found"

it's more than fair if that's the title of your 404 error page and your server is responding with a "soft 404" ("Not Found" error page and a 200 OK status code response or 301/302 redirect to a 200 OK "error page")
=8)

and no discovery dates for HTML suggestions in GWT.