When two customers check out concurrently using the same cart rule that has a quantity of 1, the cart rule can be used twice.
Steps to reproduce:
1. Start site, create two customers, create/pick test product, create test cart rule with 1 total available.
2. Both customers add the item to their carts, apply the cart rule, and perform the checkout concurrently, making sure both customers finish checkout (click the 'I confirm my order' button) as close to the same time as possible.
We have reproduced this behavior on a single machine, by performing the above steps by simulating one customer in one browser window and another customer in a second browser window.
Only one checkout successfully completes using the cart rule.
Both checkouts successfully complete using the cart rule.