I had the same issue but I noticed that it only seems to happen when I specify a callback url. The first time I tested I didn’t and I only received one email with the download link. Then I set a callback url and tested a purchase again. That time, I continued to get emails until I deactivated the plugin.
I also noticed that the plugin was printing the thank you message at the very top of the page thus breaking the custom theme installed for my client. How can I prevent that from happening? It also printed in the content area.
Thank you! I would love to see these issues corrected because this plugin does exactly what I need without looking ugly.