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Basically this is due to how shopify cart flushing system works.
Basically what happens is that the person who was able to double book has usually came back and finished checking out the booking they had in there cart..
But how? Well my app applies a cart locking system which prevents that slot from being bookable for 20 minutes and then it's released to the public again.
The thing is that if the personal who originally added it to cart comes back and finishes checking out then the booking will also go through
This will only happen if the customer comes back before Shopify flushes the cart (which they do 10 days after adding to cart)
I recommend that in BIG BOLD letters put a warning in your product description and in the calendar description warning people to double check they are not checking out a timeslot that is already sold out
To check if a timeslot is sold out have them go back to the product page of the service and click on Book Now. Once they do that they should confirm that the date/time in their cart is still availiable in the live calendar
Promise i am doing everything i can in regards to this cart flushing issue and constantly talking with Shopify senior developers over this. Some have told me they plan on rewriting this part of but it's a very delicate and complicated process.
I have spent easily over 500hrs or more just doing research on this to try and figure out how i can personally resolve this on my end but it has been a massive headache.
Think it's best if we wait for the Shopify senior developers to do their thing ;) You can read read more about this in the Shopify dev docs below
You can also try increasing the amount of time that Cowlendar locks that slot while the booking is in slot
When you set a longer cart lock duration, you significantly reduce the chances of double bookings for the same time slot. This happens because the slot is marked as unavailable to others while it's locked in a customer's cart. However, it's essential to understand the trade-off: a longer lock duration means that a slot could remain reserved for someone who ultimately decides not to complete their purchase. In this case, the slot was unnecessarily locked, potentially preventing another customer from booking it.
On the other hand, reducing the lock duration increases the chances of double booking since the slot becomes available more quickly if the first customer doesn’t finalize their purchase. This can lead to two customers booking the same slot before the system can update availability.
Finding the right balance for the lock cart duration is crucial. It ensures fewer double bookings while minimizing the risk of missed opportunities due to unused locked slots.
