lucy24 - 8:36 pm on Jul 26, 2011 (gmt 0)
have you checked your server access logs to determine who is requesting these non-canonical urls and which urls are referring these requests?
I think GWT itself has the same information. It does for vanilla 404's: there's a column that says "linked from so-and-so-many pages" and it will give you a list. Now, if the linking pages themselves no longer exist-- or never existed in the first place-- then you've got some hunting to do.
Come to think of it, are the nonexistent pages listed as both "duplicate title" and 404? I don't think anything in GWT is mutually exclusive.