That is because “send mail” brings you to the “generic mail” tab, which is sending to people registered in EME, not bookings. So probably you have multiple bookings made by the same person (which is totally legit).
So go in the “Event mail” tab, select your event and type of mail etc … which allows to send mail either per participant of the event or per booking (which knowledge of the event to replace placehoders too). If you just want to resend the approved mail, that’s possible from the “change bookings” screen.