Its generally an issue with the handshake on the server. Its reporting you didn't receive it so it continues sending it. Are you in a poor service area?
It happens to me on occasion, but not enough to be too bothersome.
I've got the Verizon Network Extender and I love it. You can give 5 numbers priority but it can handle up to 50. It covers 5k sq. ft. and you can verify your coverage by pressing #48. I paid about $200. and I've never dropped a call since I've had it. Multiple messages especially text messages are a handshake/network issue. I'm assuming you turned off the notification from the stock app if your using something else of course. If your using stock try handcent or GoSMS Pro (my preferred choice). Hope I helped.