Q: I’ve been using Messages to text with my fiance almost since it first came out, and it’s always worked fine, but recently I noticed that I have two separate threads in Messages with his name on both of them, and different parts of our conversions in each. I can send him messages in either one and he’ll get them, and he only has one messages thread on his iPhone, but messages from him seem to randomly go into one or the other. Any idea why this is happening and how I can fix it? Is there some way I can combine these two conversations into one?
A: The most common cause of this is that the other person has started sending you iMessages from a different address. iMessage users can receive messages at more than one address. It’s not uncommon for iPhone users to have at least two iMessage “addresses” — usually the user’s phone number and the email address associated with their Apple ID — and it’s even possible to use multiple email addresses.

While iMessages can be received at any of the associated addresses, one address is chosen by default to send iMessages. You can see the addresses that have been associated with your Apple ID and which ones are being used for iMessage by going into your Settings app and choosing Messages, Send & Receive.
As the name implies, the address selected under “Start New Conversations from” is the one used to initiate new conversations in iMessage, but there’s a catch: When replying to existing iMessage conversations, the sending address will be the same one that was recently used in that conversation. So in other words, if your fiance sends you a new iMessage, it will come from the address specified in the settings above, but if he’s replying to a message you’ve recently sent, it will come from whatever address you sent that message to.
The Messages app on iOS divided conversations based on the sender’s address, without regard to contact entries.
So an iMessage conversation with somebody’s phone number will be separated from an iMessage conversation from that person’s email address, even if both share the same contact record in your iOS address book. You can determine which address is being used for an iMessage conversation by tapping the “Details” button in the top right corner and then tapping the “i” button beside the user’s name. This will bring up a contact card for that user, with the iMessage address highlighted in blue.
Chances are that if you look at the two conversation threads with your fiance, you’ll discover that they’re both using different addresses. If your fiance has recently upgraded to a new iPhone, changed carriers, or even simply switched to a new SIM card, this can cause iOS to revert back to using his email address by default while the phone number is being re-registered with Apple’s servers.