The alarm in Windows Mobile has been causing a lot of problems and its a pretty well known issue. (just look on google and see)
Windows Mobile works its alarms, notifications, appointments etc. through the Notification Queue. Whatever gets entered in the queue allows the device to wake up from sleep/low-power states and do whatever it needs to do.
However, it doesn't do a good job managing this queue. If you have SKTools or any other maintenance software, you can view the queue for yourself and see. (it'll probably contain a lot of duplicate items which is why you have your problem)
You can try this tool:
http://blogs.microsoft.co.il/blogs/t...-to-alert.aspx
Which will hopefully clear things up.
I just all together ditched the windows mobile alarm and downloaded G-Alarm. So far it is pretty reliable. :P