HELP! we have 2 iphones -

Here is my problem!

We have a 3G and a 5 and every time a text msg is sent from the 5 iphone the 3 iphone receives the message as well - even though they have different phone numbers?

Then when I reply on the 5 sometimes the message only gets send to the 3G?

How do I prevent my messages on the 5 from ending up on the 3G?

Thank YOU!😕