The other thing I'm seeing is that I use categories for my calendar in Outlook but it doesn't seem as though this field is mapped in BB Calendar, at least it's not obvious to me if it is.
I'm thinking I'll redo the appts. in Outlook, re-sync, and then delete the duplicates. Looking at my alternatives, I think that's the easiest way. From there, as I've seen mentioned, it's being conscious of the account used when putting an appt. on the BB in order to avoid this in the future.