Replies land against the right event, and you stop double-booking.
About 2 mins
Two things switch on. Email means a client's reply is filed against their event instead of getting lost in your inbox. Calendar means Celestial can see when you are already busy, so a date you have taken elsewhere stops being offered to somebody new. Once it is connected, publish your availability so couples can see which dates are already taken.
These are run by the company in question and are the ones to trust if a screen looks different from what we describe.
At least one connection is listed against your account in Settings.
Sent back without connecting usually means you approved on a different Google account than the one you meant — check which account the consent screen named. If your workplace manages your Google account, an administrator may have to allow third-party apps before this can complete.
Coordinators. Connect the calendar you actually run your days from. If you keep a separate personal one, that is the one that causes the double-bookings.