I am saying that this bug isnt being caused by MyBB
When the mail gets sent, there is a list of people that the mail was sent to. Are there any duplicates?
There isn't one duplicate.
For each of the users found in your criteria, the mail function is called once, and their name is printed out in the list. So if there are no duplicates, then it shouldn't be a problem with MyBB sending it multiple times (assuming you're not clicking the button multiple times). I'm thinking it's a problem with your mail server or PHP
But isn't there another solution? Cause I had the problem also by another hoster..
Then both of those hosts have the same problem... No there isnt another solution, other than trying some new hosts
So I have to check the mailserver..
Well on this server massmail is sent once. So it has to be a server-specific problem.
This issue should be resolved in the general support forum, so I'm moving it there.