After additional investigation, we have determined that this is not a bug. There is no way to limit a purchase of differing amounts for the same product across two different users simultaneously. This is a pretty extreme edge case as you both made the purchase at the same time.

Metaphorically if a friend and I go into the grocery store together to grab a pack of soda it's perfectly feasible that I grab a 6 pack of Coke and my friend grabs a 12 pack of Coke. We both check out only to find that we've bought the same thing in different quantities.

Moving forward, the recommended approach is internal communication on purchases made on the same product to avoid this issue.

I will reach out separately to handle any refunds needed in this case.

Thank you for flagging. We were able to reproduce this issue and are working on a fix. I will follow up once we have a resolution.