I have always found this caused by a duplicate calendar entry in the BB. The problem is finding the correct entry to delete.

One fix documented elsewhere is to delete the cal database on the BB completely (possibly do a battery out reset), and resync with the PC. That presumes you synced recently and thus don't lose your latest changes.

BUT, I think there's a bug in the BB firmware. (I'm on 4.2.2.194). If I edit an existing calendar entry (change its date/time) on the BB, that seems to trigger the duplicate record. The orginal version (of the edited calendar record) is 'still about' as far as I can tell...