Overbooking is possible in version 4.9.8

[Valentina R]

To reproduce, use 2 different customers and one product with inventory 1 and the same day to schedule the appointment (hourly duration).

1. Customer1 adds a slot to the cart.
2. Customer2 adds another slot to the cart and completes the checkout.
3. Complete the Order in Admin if not already completed.
4. Customer1 still has the initial slot in cart while customer2 sees that slot as available and adds it to cart.
5. At this stage, both customers have the same appointment in cart.
6. Customer2 completes the order.
7. Customer1 also completes the order - for the same slot that customer2 just did.
8. Now, two customers have booked the same slot.

Reported here.

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[Željan Topić]

Fixed with 4.9.9. Thanks to the customer that highlighted the issue.