Elsmarc - 1:30 am on Aug 8, 2012 (gmt 0)
topstar - I can tell you one thing I am looking at. In WMT I see duplicate titles - A lot of them - in the vBulletin 3.8.7 forum on this site (where most of the "action" is). I decided to edit a lot of crawl-url-parameters in WMT which I can see is causing a lot of duplicate title pages. I don't know that this is a (the) cause, but it can't hurt.
I used to track things I did but since I stopped making any "significant" changes some years ago I have started a new spreadsheet itemizing every individual change I am making and the date.