Good afternoon and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the issue you have experienced with the event attendes list.
I would love to try to help you with this topic.
As a first troubleshooting step, could you please provide us with your system information in a private reply ? (following the instructions found in the link)
My first recommendation would be to try the following plugin instead: https://wordpress.org/plugins/duplicate-tec-event/ to see if that still copies the attendees as well.
Other than that, you can always delete all attendees in the copied event using the Bulk actions drop-down menu.
Let me know if that helps.
Have a great day!
Geoff B.