Mail programmed to send-later sends same email message multiple times

A recipient of an email that I instructed Mail to "send later" complains that he received the same email 6 times. He thinks I am doing this on purpose. But: I only sent the email to one email address and I programmed the time to send it. I did this with several messages yesterday and I am concerned that the recipients are all going to be as put out as the person who has complained.

OS 15.0.1

MBP M3