getting multiple conversations in text with the same person

Check which account each of those conversations is coming from. A user can have multiple addresses for both iMessage and FaceTime, and if they don't set their "caller ID" the same for each device, this will happen. Click into the conversation, scroll to top, click their contact info, and their "caller ID" address will be highlighted in blue on the contact info page.

 

one contact, but if you follow the above instructions, you'll probably see that an email address is highlighted for one of the entries, and a phone number for the other.

 

I'm having the same issue. My send and receive settings are consistent and so are the person's who I am receiving messages from.

 

I struggled with this same issue, both before iOS 6 and after. What FINALLY got it working for me was to have all my iDevices (and Mac) start conversations from my Apple ID. Not the phone #. If everyone does this, the iMessages will stay in the same thread.

Hope that helps you. It is frustrating.

Edit: you will also have to delete the old conversations and start again. But since I did the above, my iMessages have synced across all my devices flawlessly in a single conversation thread.