We had the same problem. At us I noticed the start/stop from the server went wrong. There were double processes running from the server. You should have 1 admin process, 1 notify process, 1 enpd process and (same as your cpu count) httpd processes.
When the other ones are more then 1, restart the system.
This receiving double e-mail reminder problem, you will only find at people who have AND event notifications on AND event & task reminders on! People who don't have BOTH activated, don't receive double reminders.
Also watchout that your test calendar system can't send e-mail notifications to your production users. But this you can see by reading the headers from the e-mail .. the system where it comes from is then different.
Hope this helps.
Greetz,
Miranda