huh. Well I can only say that I personally do not get those emails, interestingly. As I am doing it the same way as explained to OP, it seems to work for us both. You are 1. using the email associated with your Apple account, as stated, right? And 2. is it an external email or the iCloud one?
tbh I don’t know if that has any impact, but I could imagine it makes a difference. Do you have the same issues with your iMessage as OP, or are you just asking?
about the question “how does this help”.
if he sends a message to his phone number, it will show as sent and received by said phone number. I’m not exactly sure why it triples, but alr. So, if he sends it to his email, iMessage will recognize he wants to send a message to the Apple account, so it delivers. That’s how I’ve explained it to myself. Maybe it sounds dumb. Maybe it sounds really cool but is actually just nonsense. I don’t know either. I hope we’d find a solution, if you are having trouble tho. ¯\_(ツ)_/¯
quick other suggestion: iMessage / MMS and all that stuff is enabled, right?