Sending messages from an iPhone sometimes goes to email rather than to mobile
When you create a new message, it will prompt you to chose between somebody's email and phone number to create a conversation (for MMS and iMessage conversation)

You probably chose the email by mistake once (and maybe the phone number another time), so now, you have 2 conversations for the same person. One to the phone number, one to the email.

You will need to delete the email one by opening the conversation, scrolling to top and selecting "Contact". From this view, you will see either the email or the phone number in light blue. If it's the email, go back and delete this conversation.