Setster Free: Setster will always set the appointment. If you use Google Calendar and have an appointment already on Google, your clients will still be able to book that same time in Setster. The only way to avoid this is by syncing your availability between Setster and your Calendar.
Setster Pro: In the Pro Version, Setster communicates directly with Google Calendar. When you make an appointment in Google, the time slot and your availability are removed. Clients will not be able to book appointments with you if you are busy.