Has there been any development on this topic? Is there any other way to integrate other booking calendars to avoid double booking?
Hi there, @Christopher_Teibel 
Could you please elaborate on your use case? Do you mean Event Calendar or Appointment Booking widget?
The Booking widget. The calendar on the form will not show booked dates.
You’d like to get the booked slots blocked in the widget, right?
To automatically mark booked slots as unavailable, you need to connect your widget to Google Calendar:
Here’s a tutorial on how to connect your calendar: Syncing Google Calendar with the Appointment Booking Widget.
Check it out and let me know if it helped 