I understand that but why do they keep getting multiple emails regarding the same private message. I've been moderating this board for 10 years and never had this problem before. Today I've had 2 people complain about multiple emails.
If it is something that has just started happening and you have not changed anything then my next guess would be with a mail server somewhere in the chain that has been/is having problems. I have know mail servers send an email more than once.
Another bug is If you edit someone's post as moderator if someone was quoted in the post that will send an email/notification to.
The ones I listed in my first post were reported by someone else a while ago. I've submitted the one about the post notifications when editing old topics about a month ago.