Don't allow syncing same gCal globally and to staff

[Valentina R]

Would it be possible to make some check that the same calendar can't be connected globally and to the staff? If that happens, users get double appointments.

https://bizzthemes.com/forums/topic/double-sync-back-from-google-calendar/

[Željan Topić]

Issue #528 was marked as a duplicate of this issue.

[Valentina R]

This looks like a duplicate of https://bitbucket.org/bookingwp/woocommerce-appointments/issues/610/add-a-warning-or-disable-the-option-to

[Željan Topić]

Staff can sync to same google calendar, even if it doesn't make any sense. Fixed with v4.8.10.