iMessage coming up as new thread in text instead of continuation

If a person adds a new e-mail account to imessage would it then come up as a different thread or contact in my iPhone?

I received a message today from somebody I imessage and text daily, today when they imessaged it came up as a completely new text and an email address instead of a continuation of old conversation as it normally does, does this mean it is coming from a new phone number using e-mail account for messaging?

are there any other possibilities as to why this would occur?

Solution 1:

Please, check if that new phone number is in your address book. I found that having all iMessage numbers and emails in the same contact card solves this issue.