Is this from newsletters or autoresponders? I have looked at the code million times and can’t see any possibility for duplicate emails. I can’t reproduce it either. Sorry, given this I can’t help. You may need to hire someone to debug your case in place.
I know this thread, this was long time ago and the problem is fixed.
I don’t know how to say it more clear: I CANNOT HELP with your case, sorry. Maybe I am stupid and don’t see the bug. I am doing my best to find it, but can’t. I can’t reproduce the problem either. So you’ll need to either switch to another plugin or hire a better programmer than me to solve it.
There is no increment like the one you describe. Users are selected from the database ONCE and then the script goes through the fetched array ONCE and sends email to each. It’s virtually impossible to have multiple emails sent. I don’t know how on earth it happens to you, but I have looked millions times at the code and can’t see any bug.