We’ve had multiple cases of coupons being used more than once when the checkbox “Check this box to limit this coupon to the first purchase of a customer only. (Verified by billing email address or user id)” was checked.
– forward the order confirmations of one of those email addresses
– send me the coupon code and site url so I can test it.
– temporarily enable settings > Extended Coupon Features > Debug mode
You can use this email address: admin at soft79.nl.