Good morning,

Unfortunately, we are still getting issues with customers booking the same slot. 3 within the last month. 

Surely you could simply create a new 'status' ('payment pending' perhaps) which temporarily prevents an appointment from being booked by a third party? If a second customer completes their order for that same slot while it has that status it would then force them to either chose an alternative slot or offer a refund? It could then revert after 10-15 minutes (called by a cron perhaps).

How a website with higher traffic copes with this issue I don't know? This is a fix, not a feature. My client's reputation is on the line here - and yours. You have an article regarding how your plugin helps prevent 'double bookings'. At this moment, I'd consider taking that article offline if I were you.

Many thanks,

Dave