Messaging problem

You could possibly check the following

• You have earlier texted separate texts to the same contact using their two different numbers or email IDs
• Open both the texts and see which number has been used for texting.
  • Open the conversation
  • Click "i"
  • Click ">" to open the Contact
  • Check (* Recent) under your Contacts
• Do you have another contact maintained separately under list of Contacts App for these two numbers. If yes, delete one.
• Does this contact have been linked to some Social Networking Contacts such as FB, LinkedIn, Yahoo etc
  • Open Contacts App
  • Open the Contact
  • Click Edit
  • Scroll Down to the bottom
  • Linked Contacts
• Has this contact changed their carrier recently?

I can only think of these probable possibilities as of now.