I just got an iphone 5s. I was sent a group text message from a friend. When I responded, it showed my response in the group message, but then chimed and showed a new text from myself which was the exact thing I just sent to the group. So I send a response, but instead of sending it just to the other people in the group, it also send it to me? Then when I read through the group messages, I duplicates and it gets confusing.
Several things can cause this. I would need more information to give you an accurate answer. If you are using iMessage it could be that a number or email in the group is somehow associated with your contact information. Go to Settings>Msessages and make sure that only one phone number and AppleID is linked to your account. Check the group and make sure you only see your email or your phone number in the list, not both.
If it's SMS contact your carrier as that is controlled by the carrier programing.