Why am I receiving multiple reminders for the same event from my Yahoo Calendar?

I've checked the reminder settings for the events and they seem okay (send reminder 3 days before, or 1 week before, etc), but instead of getting just 1 reminder notice (as set up) I'm getting 4, 5, or 6 sent for the same event within 1 or 2 minutes! All the events involved are set up to repeat annually (birthday, anniversaries, etc).