Thank you for your feedback. We have updated WiFi Guard so that it does not send duplicate e-mails.
Now, if WiFi Guard detects the same set of unknown devices over several runs, it will only send one e-mail. If the set of unknown devices changes, it will send another e-mail. The update can be downloaded here.
WiFi Guard will not send identical e-mails, but if the list of detected devices changes, new e-mails will still be sent.
For example, if WiFi Guard finds an unknown device AABBCCDDEEFF, it sends an e-mail. If it finds the same unknown device again, it will not send more e-mails about it (until you restart the application).
However, if any other devices come or go, the list of detected devices changes and new e-mails will still be sent, which will of course include that AABBCCDDEEFF device if it is still there.
You can mark those devices as known (if you know they are legitimate), so that they are recognised as such and don't trigger e-mail alerts.