Unfortunately this is misleading and I don't think this is the problem here. To automatically handle IPv4 and IPv6 traffic on most systems, Certbot's standalone plugin first attempts to bind to the port for all interfaces using IPv6 and then bind to the port using IPv4. On most Linux systems, binding using IPv4 fails as IPv4 traffic is routed to the IPv6 port, but since this isn't the case on all systems like the BSDs, Certbot tries with both protocols and continues execution if at most one fails.
It's not clear to me what's causing the challenge to fail, but I think what's causing requests for cupon2go.com to succeed is authz reuse. Let's Encrypt has already validated that domain so it's not checking again. I suspect if you run sudo certbot certonly --standalone -d cupon2go.com --dry-run, the command will fail.