IPhone Contacts Won't Delete Permanently?

When I delete a contact from my iPhone 4s, most of the time the contacts number will not delete permanently and the number will show up when I go to send a text message. For instance, my mom's number is (123)45-7899 well if she gets her number changed and I go into my contacts to change the number her new number will show up in my contacts and the old number won't show up in the contacts anymore. But when I go to text her new number, both numbers show up, her new number shows up on top, and her old number shows up under that as though it is a separate contact. How do I stop this from happening? And get the duplicates from showing up? It really is more annoying than it sounds when every time one of your contacts changes their number, and the old one never deletes out of your phone no matter what you do.

I've had the same issue. I think it has something to do with syncing your contacts with an e-mail account. When you go to text it's pulling contacts from your e-mail account which you may have inadvertently synced with your phone. Hopefully someone else can confirm that for me.