iTunes 8.1 creating duplicate entries in iTunes, iCal, Safari and Address Book

Some iTunes 8.1 users have been suffering from a nasty little bug that creates duplicate entries in iTunes, iCal, Safari and Address Book.

The incident appears to be somewhat isolated and not an epidemic, but anecdotal evidence of duplicate applications, songs and calendar events can be found in Apple's discussion boards. Duplicates are notoriously tough to diagnose, but it appears to be an iSync problem.