Double bookings in appointments+, using google api?

About once a week we get a double booking – the first one is made directly to gcal (api sync) atleast a few days before, and then someone makes a reservation through website to a overlapping time spot (it was available even though there was google calendar event!). I checked that all the other events in gcal have also been reserved in appointments plugin correctly (I’m not sure about these double-book ones, as it doesn’t show past gcal reservations). There was no difference how these reservation were made. Can you help me? We cannot continue using this plugin if double bookings continue to occur.