The pending edits are stored in the Session object - is it possible your session persists across app restarts?

AFAIK the duplicate key issue relates to the 'key' used internally in the collection of Page Url Objects, not in the database.  It shouldn't be possible to get a duplicate key back from the database (because it uses an identity column), so I would expect the problem to be something related to an in-session edit that has caused it.

I know I'm being vague, if you can isolate how you think you caused it I will be able to find it that much quicker.

No clearly not a session problem.  Can you do a select of the rows in the TabRedirect table and email them to me?  Just stick them in a spreadsheet or a text file, and reply to the notification email that you receive from the forum.

Also, can you copy in a copy of the exception that you get?   I just need to try and work out which entity is tripping over a duplicate key.