When you sync an appointment that is recurring from your phone to Outlook, it doesn't notice that Outlook already has the recurring event and the particular instance.

When you delete an appointment from Outlook that has already been synced to the cellphone, it will fetch it from the cellphone again and recreate it in Outlook. To fix that, we'd need to remember all the items currently on the cellphone (at the end) in a file and the next time at the beginning, check whether any of them are in the file but not in Outlook now. If not, delete them from the cellphone.