Allyson Kazmucha writing for iMore:
Along with receiving iMessages at your phone number on the iPhone, you can receive iMessages at your Apple ID email address on iOS and OS X as well. But what if you’ve got more than one email address and you want to be able to use all of them with iMessages?
A friend of mine recently sent me an iMessage from her computer, then later sent another message from her iPhone. One message came through from her e-mail address while the other came through from her phone number. Here’s how to merge the two.