Ah, thanks for the pointer. I will fix that in the next update. ??
What is not clear about the filter? I had hoped I constructed a (hopefully) clear helper text. ??
If you allow double bookings, you can choose to send an email to all bookings. Then double bookings get double emails, because they booked twice (or more). Yet, every email will process the placeholders for each email. So a double booking gets an email for booking #1 and #23 for example. Booking details might be different per booking (the booking ID, number of tickets, total price, etc).
If you filter by email, double email addresses are filtered out. That leaves less, unique email addresses. So even if someone booked 5 times, they will only get 1 email. Because only the first email is used (the rest is filtered out), only the booking placeholders for that first booking will be processed.
So, if you are NOT using booking placeholders in the email, you best filter by email. Otherwise double bookings will receive multiple and identical emails.