In the email/register endpoint, the transactional Welcome email is sent regardless of whether the user was already subscribed. See if there's a way to add a check to only send the welcome email if this is a new subscriber.
Actually, the main problem is that the "Submit" button can be clicked multiple times, resulting in multiple requests being sent. This can be trivially fixed by disabling the button until the callback is received.
In theemail/registerendpoint, the transactional Welcome email is sent regardless of whether the user was already subscribed. See if there's a way to add a check to only send the welcome email if this is a new subscriber.Actually, the main problem is that the "Submit" button can be clicked multiple times, resulting in multiple requests being sent. This can be trivially fixed by disabling the button until the callback is received.