On March 22nd, 2018 we went for a Sale on our Magento 2.1.5 live site deployed on an AWS server m4.4xlarge. On 12am sale started with 1500 online visitors and in the next 3 peak hours, there are some duplicate orders. The amount, items, billing/shipping everything is same even the quote_id (verified from database table sales_order) is same. Entity_id and increment_id is changed only.
We never face this same issue before Sale or after Sale, even for the next three days of sale there is no such duplicate issue.
Duplicate orders are only for guests with cash_on_delivery payment method.
I checked the loader issue but that is not the case since "Place Order" button gets disable and full-page loader appears as soon as you click the button for the first time. I'm not able to replicate the issue. Any help?