If you see duplicate appointments showing up on your calendar this is probably a sync issue.
This typically occurs when you sync the SimplePractice calendar twice to the same external calendar.
iCal & Google Calendar
With iCal and Google calendar, you only need to create a sync once and it will display the SimplePractice sessions on all of your devices where you access your iCal or Google calendar- on your phone, your desktop, your tablet, etc. So this means you only need to sync your SimplePractice calendar with one device and it will be sync'ed with all of them.
To fix this issue, delete one of the SimplePractice calendar syncs you've created.