an item with the same key has already been added

Using MOSS 2007 with Aug 2009 cumulative.

I have a site collection with publishing turned on.  When I go to the site settings page and click 'Content and Structure' under Site Administration, I get the lovely error page with the single message 'an item with the same key has already been added.

The users have access to SP Designer.  Where should I look for modifications?

Thanks.
GregM