Is the message deleted from her phone? Have her clear her sent messages or conversations. Then pull her battery with the phone on.
Put the battery back in and restart the phone, then dial *228 and choose option 1. This will reset all the network settings and hopefully clear the glitch that has the message(s) resending....
Great suggestions SuzyQ! Those suggestions may definitely clear up this issue.
How irritating to get the same message over and over again, that happened to me once due to low coverage on my phone. I fixed this problem by dialing *228 pressing send then option 1. This forced my device to look for all the towers in my area.
If your daughter's device is having memory issues on her phone it may be sending out duplicate text messages. In this day and age people use text messaging a lot. She may have low memory and not even know it.You can check her memory by using these steps Main Menu>Settings>System>Memory. This will tell you how many Free MB's(megabytes) the device has available. If less than 15 MB she will need to get rid of some old messages,pictures (if not on a memory card), and applications.
If this doesn't fix the issue please let me know and I will further troubleshoot with you.